The "golden rule" that I was taught is "half your age plus seven." So if your 20, you can date a 17 year old. If your 40, you can date a 27 year old, etc... I know that it makes no sense as a rule, but it does seem to fit together ages that don't feel horribly creepy[footnote]at least not to me personally, I can't speak for everyone else.[/footnote], so I guess it's alright. In the end, as long as both parties are of age they can do what they want. No matter how weird or normal it may seem to others.