It all depends on you and how you feel about it.
For me personally too young should be when It's illegal for you to be with them.
For example I was with a 36 year old women not to long ago and neither of us had a problem with it, but I was in a club the other day and my buddies introduced me to this girl and when I asked her how old she was she said 15.I was outta there as soon as possible.but as I said befor It all depends on You and your personal taste some won't go more than a 3 year difference between age and there are some that really don't care.