[robocop]

A: 21 is super super young and means nothing. You thinking you're too old at 21 is almost comical, not to be insulting, just to snap you out of it. I had a buddy who toured as a musician until he was 32 then went to med school, my aunt just became a lawyer at 45. A buddy from high school who is 27 just started his second undergrad because he decided he's going to be a vet.

B: If you want to become a doctor you should take these steps, my roommate got in and this what he did:

i:work towards another degree in something that has a future, something that interests you that you can parlay into a career in the worst case scenario that you don't get in (this is generally something in natural sciences)
ii: while in your second degree work your tail off and keep a 90+ average
iii: get the mcat books early in the summer, get a slack job and study your balls off the entire summer, the mcat will be harder than you think
iv: you will most likely have to take one year of nat. sci prereqs, then take the mcat late that summer, then take another year so you can apply
v: get tons of experience with the interview process, this is generally the hardest part, a handful of my friends didn't get accepted because they failed the interviews while having amazing marks and acing the mcat
vi: get a good written reference from a family friend or business associate who is a certified professional
vii: if you do well on the mcat and your marks are high apply and get ready for the interview, what they are really testing you on is your ability to form rational decisions in high stress situations. I had a friend who had a 98% average in university but failed the interview 3 times because he had very little critical thinking ability.

this is only a blueprint but I've seen maybe 40 people try and of those 40 only 3 got in because the rest of them screwed up one of the steps along the way. Good luck, you can do it if you want to, you just have to want it bad enough