I'm 22 either and I made that choice 7 months ago and I'm happy I made that choice but...
That's a tough choice.
You have to weigh the pro and the cons:
Pro:
-Might see better regrowth with rogaine at early stage of hairloss(22)
Cons:
-Life treatment
-Have to use it mornings and evenings for 1 year. Then after this year, 5 times a week to maintain your regrowth
-May have side effects(which is rare though)
I think it all depends on how you feel about your hairs now. If it's not best but that you are still happy with it, keep it like that. If you really feel that it is terrible or going terrible, you might want to start the Foam.
If you see regrowth with propecia, maybe it's better to stick with it for a whole year to see its full effect on your head.
Just my 2 cents,
Good luck mate!