Finance media loves scolding the young — the coffees, the festivals, the takeaway boxes of doom. Most of it is noise: the twenties' small pleasures are cheap, and joy has value the spreadsheets ignore. But hidden among the harmless 'mistakes' are a few structural ones whose costs compound for decades. The skill is telling them apart.
The honest ranking surprises people. The latte is nowhere near the top. The quiet, boring, paperwork-shaped errors are.
The mistakes that barely matter
Spending on experiences while young and mobile — travel, scenes, misadventures — converts money into identity and stories at the one age the exchange rate is spectacular. Suboptimal fund choices within sensible investing barely register over forty years next to not investing. Even a modest, contained splurge habit is fine inside a system that saves first.
The pattern: mistakes of degree are cheap. If the structure is right — spending less than you earn, saving automatically — the details forgive themselves. Perfectionism about details is itself a classic twenties error: it delays starting, and starting is the entire game.
The mistakes that compound against you
Number one, by miles: not starting to invest, because the twenties' money has the longest runway and every delayed year is the most expensive year to lose. Number two: expensive revolving debt — carrying credit-card balances normalises paying triple for everything and trains the budget to serve interest. Number three: no emergency buffer, which converts every mishap into new debt and keeps the whole cycle alive.
Then the paperwork tier, invisible but heavy: skipping employer retirement matches (declining free money), remaining uninsured against genuine catastrophe, and ignoring credit health in systems where it prices your future housing. None of these feel like decisions at the time. All of them are.
The decade's real assignment
The twenties' financial job is not wealth — almost nobody builds it then — but architecture: the automatic transfer that makes saving ambient, the invested account that makes compounding start, the buffer that makes bad luck boring, the clean debt habits that make future borrowing a tool instead of a trap. Modest amounts, correct structure.
And self-forgiveness has a place in the system. Everyone's twenties contain a financial embarrassment or two; their function is tuition. The only unforgivable version is the one that never gets converted into a rule — automate the lesson, keep the story for dinner parties, and let the decade do what it is actually for.
Ranking errors by how long they last
The useful way to sort financial mistakes is not by the amount involved but by how long the consequence persists. This produces a ranking that differs sharply from the one most advice implies, and it explains why so much attention is directed at things that barely matter.
A one-off overspend, however large it felt, resolves within months and leaves nothing behind. A recurring commitment entered into for years persists for exactly as long as the commitment. A missed decade of compounding persists for the rest of a working life. A choice that shapes what career you are in persists longest of all.
Applying this ranking immediately demotes most of what gets discussed. The purchases people feel guilty about are almost all in the first category. The decisions that actually determine where someone stands at forty are in the third and fourth, and they are made with far less deliberation because they do not feel like financial decisions at all.
The errors that are basically fine
Some of the classic twenties mistakes are worth defending. Spending money on travel, on experiences with people you will not always have access to, and on the ordinary social life of that decade is frequently criticised in financial writing and is a reasonable use of money that will never buy the same thing again.
Changing direction, leaving a job that was going nowhere, taking a lower-paid role to get into a field you wanted, and periods of relatively low earning while working something out are similarly presented as costly and are largely investments with a long payback. The alternative — staying in the wrong thing because leaving costs money — is considerably more expensive over a career.
Small inefficiencies belong here too. Suboptimal savings accounts, funds that were not the cheapest available, a period of not investing while working out what to do. Each of these has a cost that is real and small, and the energy spent regretting them is better directed at the things in the next section.
The errors that genuinely compound
A shorter list does lasting damage. High-rate revolving debt allowed to persist is the clearest, because the compounding runs against you at a rate no investment reliably matches, and a balance carried through the twenties can consume the entire capacity to save during the decade when saving matters most.
Vehicle finance is the most common specific instance, being a large multi-year commitment against a depreciating asset, entered into at an age when the payment looks affordable relative to a first real salary. It is the single decision most likely to prevent someone from building anything during their twenties.
The third is inertia on employer pension arrangements: not joining, contributing below a match threshold, or leaving contributions at a default set years earlier. This costs nothing to fix, is invisible while it is happening, and is worth a very large amount over a career because of exactly the compounding described elsewhere on this site.
The one that is not about money at all
The most consequential financial decisions of the twenties are frequently career decisions that nobody categorises as financial: what field to be in, what skills to build, which employer to join, whether to move somewhere with a better market for what you do.
These determine the income that every other decision operates on, and their effects are permanent in a way that no spending decision is. Someone who spent their twenties in a field with poor prospects, saving diligently, is in a worse position than someone who spent the same decade building earning power and saving less.
This is not an argument for chasing money into work you dislike, which has its own costs and frequently does not last. It is an argument that the choice of what to do, and the deliberate development described in the skills article on this site, deserve at least as much attention as the savings rate, and that they usually receive far less because they do not present themselves as money questions.
The mistake of doing nothing while working it out
A specific and common pattern deserves separate mention: postponing every financial decision until things are more settled. This is entirely understandable in a decade characterised by change, and it is expensive because the delay applies to precisely the years that compound the longest.
The version that works is to make the reversible decisions immediately and defer only the irreversible ones. Starting a small automatic contribution requires no certainty about the future and can be adjusted at any time. Joining a pension scheme costs nothing if circumstances change. Building a small buffer is useful regardless of what happens next.
What genuinely should wait are the commitments that are hard to undo: property purchases in a place you may not stay, long-term financial products with lock-in periods, and any arrangement that assumes a stable income you do not yet have. Distinguishing these from the reversible ones removes most of the reason to postpone everything.
What the decade is actually for
Framed positively rather than as a list of errors, the twenties have three assignments and they are unevenly weighted. Establish the habit of spending less than you earn, at whatever small scale is possible, because the habit is what persists rather than the amount. Build earning power deliberately, since the increases achieved here apply to everything afterwards. And avoid the small number of commitments that would prevent the first two.
Everything else is optional. The optimisation, the fund selection, the tax efficiency, the detailed planning — all of it matters more later, when there is more to optimise, and all of it is a poor use of attention in a decade when the amounts are small and the trajectory is being set.
The reassuring implication is that the assignment is short and mostly achievable regardless of income. The discouraging one is that its most valuable component, the years of compounding, is the one thing that cannot be recovered later at any price. Which is the whole argument for starting badly rather than not starting. None of this is financial advice, and every situation is different.
Advice from people whose twenties were different
A great deal of guidance aimed at this decade was written by people whose own twenties occurred under materially different conditions: different housing costs relative to income, different employment structures, different pension arrangements, different levels of student borrowing. Some of it transfers and some does not, and the parts that do not are the parts delivered with the most confidence.
The advice that transfers reliably concerns mechanisms rather than amounts: compounding works the same way, the gap between earning and spending still determines the outcome, expensive debt is still expensive. The advice that does not transfer concerns benchmarks — what proportion of income housing should take, what should have been saved by a given age, when property purchase becomes reasonable.
The useful filter is to ask whether a piece of guidance describes how something works or asserts what a number should be. The first is durable. The second was calibrated on a set of conditions and should be recalculated against yours rather than accepted, particularly when it is being used to conclude that you are behind.
Repairing the ones you already made
Anyone reading this at the end of the decade rather than the start will recognise items from the damaging list, and the relevant question is what can be undone. More than people expect, and the ordering is fairly clear.
Expensive debt can be cleared or refinanced, and doing so is the highest-return action available. A vehicle commitment can sometimes be exited at a cost that is less than continuing. A pension contribution can be raised immediately, and in some systems past gaps can be filled retrospectively. None of these recovers the lost compounding, and all of them stop the loss continuing.
The thing that cannot be repaired is the time, which is precisely why the appropriate response is action rather than regret. A decade of compounding forgone is a real cost and it is bounded; a second decade forgone while feeling bad about the first is the version that does lasting damage. The correct move is always the same one: start now, at whatever scale is possible, and stop calculating what an earlier start would have produced.