Compound interest. Buy into a total market index fund that will get you more or less 7% on average a year. Let’s say you have a $100 extra per month (doesn’t actually matter whatever you can afford).

Sally from age 20-30 puts her $100 in every month. At the end she has about $16000. She stops adding anything, but keeps that money invested. By 65 she has $170k.

Jeff doesn’t start investing until he’s 30, but he’s consistent and does the same thing, $100 a month from 30-65. He ends up with about $165k.

What that means is Sally made more money than Jeff even though she did the same thing for 10 years that he did for 35. She just started earlier.