I came to the horrible realisation that the freshers at my university seemed like children when I was in my final year.. I'm 23, now, and I couldn't date an 18 year old, just off the bat.
Naturally, if I'd got to know well them without having a relationship in mind and then a relationship organically evolved, I could most likely pull myself round to that score, having started seeing them just as a person, rather than a young person.
Last bit of slap and tickle I had was with a lady of 26, though. I think I'm enjoying the benefits of looking about 30 (due to my beard).