Okay, there's a VERY, very POPULAR rule out there.
"Take half your age, add seven."
And for the most part, I think it's accurate. But before DEDEDE comes in here saying society is wrong to put such labels, let me clarify one thing. I'm not a believer that certain people shouldn't be together because of their age... but well, there is a maturity difference. And you'll realize it, one way or the other, if you're on the older side or younger side, that it makes relationships difficult to work. So the half your age/7 year rule actually just speaks to the maturity/stages of life. I'm looking at it in that aspect, rather than the morality bullshit that is usually the first thought when this topic is brought up.