Compound interest: why starting at 25 vs 35 changes everything

Compound interest is the closest thing to magic that personal finance has — and the closest thing to a tax on procrastination. The maths is simple, but the consequences are extreme: a decade of delay can halve your final pot, and most people don't realise it until the decade is gone.

What "compounding" actually means

Simple interest pays you on your original deposit. Compound interest pays you on your deposit plus all the interest already earned. Each year the base grows, and the interest the next year is calculated on the bigger base. Year 1 looks unimpressive. Year 30 doesn't.

At 7% a year, money roughly doubles every 10 years. So £10,000 today becomes about £20,000 in 10 years, £40,000 in 20, £80,000 in 30, and £160,000 in 40. The last decade alone adds more than the first three combined.

The 25 vs 35 problem

Here's the example that does the work. Two people both invest £300/month at a 7% real return until age 65.

Alex starts at 25 and invests £300/month for 40 years. Total contributions: £144,000. Final pot: roughly £790,000.

Jamie starts at 35 and invests £300/month for 30 years. Total contributions: £108,000. Final pot: roughly £365,000.

Alex paid in £36,000 more over the extra decade — and ended up with over £400,000 more. The early money had longer to double, and double again. That's not a 25% advantage; it's more than double the result.

Why the last decade does the heaviest lifting

People assume that years of investing contribute roughly equally to the final pot. They don't. The pot grows exponentially, so the biggest absolute gains come at the end — but only if you put the years in. Skip the early years and the late ones have a much smaller base to work with.

Put differently: an investor who starts late can never "catch up" by saving harder, because what they're missing isn't money — it's time.

What rate of return is realistic?

For long-horizon equity investing, a real (after-inflation) return of 5–7% is a defensible long-run assumption. That's after stripping out inflation, so the resulting figure is in today's spending power. Cash savings have historically returned 0–1% real, sometimes negative. Most of the wealth in the example above comes from being in equities, not from being clever about which equities.

Three rules that follow from the maths

Start now, even if it's small. £50/month at 25 beats £200/month at 40 over a working life. The amount matters less than the years.

Leave it alone. Pulling money out resets the clock on whatever you withdraw. Selling in a downturn locks in losses that compounding would otherwise have unwound.

Costs compound too. A 1.5% annual fee versus a 0.2% index fund fee doesn't sound like much, but over 40 years the high-fee option can quietly cost you a third of the final pot. Cheap, broad, and boring usually wins.

The other side of the same coin

Compounding works against you on debt. Credit card balances at 20%+ APR double in under four years if left untouched. The same exponential curve that builds wealth slowly destroys it on the wrong side of a loan. If you're carrying expensive debt, paying it off is often the highest-return investment you can make — guaranteed, tax-free, and immediate.

Master compounding on both sides — for you on investments, against you on debt — and most of personal finance falls into place.