The observation that starting earlier matters more than contributing more is repeated constantly and is rarely demonstrated. The arithmetic is worth seeing.
This explains a mathematical relationship rather than advising anybody on retirement planning, which is a regulated activity.
The mechanism
Compound growth means returns are earned on previous returns as well as on the original amount.
Which produces growth that accelerates over time rather than proceeding linearly.
The consequence is that the earliest contributions have the longest period to compound and therefore contribute disproportionately to the final total.
The demonstration
The standard illustration, which is worth working through.
Consider two people, one contributing for ten years starting early and then stopping entirely, and another starting ten years later and contributing continuously until retirement.
Under typical assumptions about growth rates and periods, the first person frequently ends with more despite contributing for a fraction of the time and a fraction of the total amount.
The reason is that the early contributions had an additional decade to compound, and a decade at the end of a long compounding period represents a large proportion of the total growth.
The specific figures depend entirely on the assumed return, and the qualitative result holds across reasonable assumptions.
The doubling relationship
A useful mental tool.
A commonly used approximation estimates the time for an amount to double by dividing seventy-two by the annual percentage return.
At a return of six percent, doubling takes roughly twelve years. At nine percent, roughly eight.
Which makes the effect of small differences in return over long periods visible, and it is the same reason charges matter so much.
The approximation is rough and adequate for understanding the shape of the relationship.
What the illustration assumes
Worth being explicit, since these illustrations are frequently presented without caveats.
A constant annual return, which does not happen. Real returns vary substantially year to year, and the sequence matters, particularly near the end of an accumulation period.
No charges, which is unrealistic and which as discussed elsewhere reduces the outcome substantially.
No inflation, so the resulting figure is in nominal terms and represents less purchasing power than it appears.
And no tax, which depends entirely on the jurisdiction and the account type.
Which means these illustrations demonstrate a principle rather than predicting an outcome, and treating them as forecasts is a misuse.
Sequence of returns
A risk that the smooth illustration conceals.
Because contributions and withdrawals occur over time, the order in which returns arrive affects the outcome even for the same average return.
Poor returns early in an accumulation period matter less, since the balance is small. Poor returns immediately before or after withdrawals begin matter considerably more.
This is a well-documented consideration in retirement planning and is one reason asset allocation is commonly adjusted as a target date approaches.
Employer contributions
Worth noting because the arithmetic is unusual.
Where an employer matches contributions, the match represents an immediate addition that then compounds alongside everything else.
Which means declining an available match forgoes both the contribution and its compounded growth over the remaining period.
The specific arrangements vary enormously by jurisdiction and employer, and understanding what is available is worth doing.
The practical implication
What the arithmetic actually supports.
Time in the market matters, and starting earlier with smaller amounts is generally more effective than starting later with larger ones.
Small differences in charges compound in the same way and in the opposite direction.
And the certainty of contributions and charges contrasts with the uncertainty of returns, which is why those are the elements worth focusing on.
What none of this establishes is what anybody should actually do, which depends on circumstances, timescales and risk tolerance, and is a matter for regulated advice.
Regular contributions versus lump sums
A related question the illustrations rarely address.
Contributing regularly over time means each contribution has a different compounding period, so the effective growth applies to a rising balance rather than to a fixed one.
Which produces a different and generally smaller result than an equivalent total contributed at the start.
Regular contribution also means buying at varying prices over time, which some describe as reducing the risk of committing at an unfavourable moment and which does not improve expected returns.
For most people the question is academic, since income arrives regularly and contributions follow it.
Real returns rather than nominal
The adjustment that makes long projections meaningful.
Illustrations in nominal terms produce large figures that represent considerably less purchasing power decades later.
Projections using a real return assumption — the return net of inflation — produce smaller and more interpretable figures, expressed in today's money.
Which is how regulated projections are generally presented in several jurisdictions, precisely to avoid the misleading impression that nominal figures create.
Anybody running their own calculation should decide which basis they are using and be consistent, since mixing the two produces nonsense.