I'm 21. I think I'd find it hard to be interested in anyone younger unless they really stood out as above their age.
I would probably date up to around 40 years old, they would have to be pretty special/be great in bed. I've been with a 32 year old and have been emotionally interested in a 37 year old. With the latter, I didn't know his age specifically, it simply didn't matter. I don't feel that age alone matters.
My partner is 26, though.