How come different races/ethnicities seem to age so differently?

I'm only in my 20s, but I've noticed among my parents' friends (all in late 50s and 60s) that the white people have been steadily aging visibly for years while the black ones have looked fairly young this entire time, but now a couple are suddenly starting to look very old. They don't have too many Asian friends, but it seemed like they suddenly started looking old several years ago after looking younger than everybody for so long. There are a couple of exceptions of course, which I'm sure depends on people's genes-- but for the most part this is what I have noticed.

Just to be clear, I personally don't think there's anything wrong with aging. Obviously I'm not looking forward to it myself, but I think we all need to accept the fact that we will all go through it unless we die young. I'm just interested in why it is different races seem to age differently. Anyone know of any studies done on this?