I started investing at 18 with $500 in a Roth IRA, mostly by accident — my dad set it up and I didn't really understand what it was. I'm 34 now and that account, with contributions through my mid-20s, is worth $48,000.

If I had started my 401k contributions at the same time instead of waiting until I was 26, my models suggest I'd have roughly $30-40k more today just from the earlier start date. That's the compound interest demonstration I use when talking to young people.

The math isn't complicated: money invested at 22 has 43 years to compound at 65. Money invested at 30 has 35 years. That 8-year difference at 7% annual return means the earlier dollar is worth roughly 1.75x the later dollar at retirement. Starting early isn't everything, but it's something you can never get back.