It's all about the maturity level, really. Personally, I'd go for almost anyone, so long as their maturity level matches mine. There are many other factors that narrow that field down, but you know.
Realistically speaking, I think my range at this point would be 16-36 (10 years younger to 10 years older). There aren't many teens that are mature enough, but it IS possible. And I generally find people older than 35 or so to be too cynical for my tastes.