I agree, I would like an age slider. It might make the beginning quest more awkward, though. If you make your character like 60 at the start, it'd be a bit strange to have your younger siblings still be kids. But... I kind of think that mild weirdness does not outweigh the benefits of getting your character the age you want.
The character creation could use slightly more variation, anyway. I would like a proper ginger hair color, so that I could approximate myself a bit more. And sometimes I feel like most of the female companions you come across kind of look alike. But those are separate issues, I suppose.
By the way, I think the brother you start with is actually younger than you, as well. When I look him up in the encyclopedia, he's said to be age 25, and I start out as age 30 or 31.