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?

DCA monthly into index funds and DO NOT pause it if you can afford to. It doesn’t even have to be a large amount. I missed out on so many years where I could’ve just hit the bullet and ate out less or something like that.