August 8, 2006... How young is too young?

A lot of times I have  young guys that show more than a passing interest in me. 
I have to admit it is a bit flattering to have guys half my age hitting on me, though we all know WHY most of them are doing it.
Then there are a few that fall into a more serious category.  They seem to have a genuine interest in establishing a real relationship.  That's when I start getting confused.
Let's say he shares a lot of my interests.  Likes the same music, can hold a conversation with me, and is really an interesting person.  He is someone I think I wouldn't mind hanging out with.
But he's younger.  HOW MUCH YOUNGER IS TOO YOUNG???  Anyone who is 8 years younger than me is closer to my daughter's age.  I don't  know that I'm comfortable with that.
It is said age is only a number, and that's not entirely true.  It is a number, but it is  not only a number.  There are differences in perspective based on how long you have lived.  Understanding comes with age. 
How can I decide how young is too young?  Perhaps I need to look the other direction?  If  a fellow is older than me, how old is too old?  Perhaps that would clarify things for me. 
I would date a guy 10 years older than me with no thought about it.  15 years ....  if the fellow shared a lot of my interests.  20 years..... that's pushing it.  I can't see me dating someone that's 70 at this point in time.  I won't categorically say no, but I do doubt it.
So?  I guess I would consider a fellow who was 43 or so, but any younger?  Nah.  I think that would be pushing the limits of rationality