Compound interest: why starting early beats saving hard
Interest on interest is slow at first, then suddenly it is most of the pile.
Compound interest means your returns themselves start earning returns. The effect is invisible in year two and dominant in year twenty.
100 € a month at 5% is about 15,500 € after ten years — but roughly 41,000 € after twenty, and 83,000 € after thirty. The later decades do the heavy lifting, which is why the early ones matter most.
The same mechanism works against you in debt: unpaid credit-card balances and overdrafts compound too, just with a worse sign.
The practical rule: automate a fixed monthly transfer to savings or investments, however small. Time in the market is the ingredient you cannot buy back later.