I would say, don't bring up the fact that you are young. Focus on you being African-American and add something extra to it, something you like to do, a hobby, a serious disease you had to overcome, financial problems, etc.. You just want to distinguish yourself from other applicants. But being young, it's something i wouldn't want to accentuate in my app.
EDIT: DON'T focus on being African-American (i changed my mind after reading the entire post), if you brought that up in your PS, that's already enough. Talk about maybe your struggles, your motivations for pursuing medicine, your view of medicine and what you want to do. In my PS, I did not mention that I was asian/brasilian at all, but I did so in the additional essay mention that and how having to move from place to place (never staying at a place for more than 2 year) and suffering through discrimination (mainly in Brazil) and always having financial concerns (not having a matress until I was 16ish) made me stronger, etc. Then I also mentioned that I would like to one day return to Brazil and work with indigenous people and learn about their use of Amazon plants for healing. Something weird like that would suffice.
