3-5 years younger or 2-3 years older than me...
More than that there is just too much of a generation gap... I like having something in common with the person I am going out with.
I'm 35 and dated a 24 year old a few weeks... felt like I was babysitting her... even the sex wasn't that great (there is a lot to be said for an experienced woman in her 30's!).
I guess "people in your age range" is the best answer to the question "how young is too young".
Red