Many people answer the "what do you wish you knew in your 20s" with "take advantage of compound interest", but never elaborate. If you are this person: 

What do you mean by that? Are speaking purely financially? If so, exactly what financial decisions would you have made differently?

Here is what I think it means:

Everything compounds.

So financially, yes, you could make a lot of money down the road, but start putting aside some money into index funds. 

Health wise, get into good habits - have a consistent exercise regime.

Same with knowledge. Learn new things + make it a habit.