First off, just to state this, I am a Sophomore in High School. Now, don't ignore me because of that. I actually have seen some ridiculous relationships, and wanted to explain what I feel are "boundaries".
OK, to start with, is School. If both of you are out of school, only my last criteria applies to you.
Second up is the school year thing. I am in tenth grade, and I am still young for my grade, 'cuz I was in Nebraska before I came to Georgia, and there the age laws for school are less strict. Now, there is a freshman my EXACT age. We checked. It's crazy. People always say we act alike, but then, after getting to know us, I turn out to be more mature (go me. I'm also very modest, and rule most of the world). When someone first told us this, I was surprised, and asked the freshman what he thought of this (we're just friends, not even really good ones). He said something that, although painfully obvious, stuck in my head for a while: one year in school makes a lot of difference. It's true, too. I mean, there's this one senior that I could've gone out with, but I decided not to. Why? Because of age difference (only two years), and the fact that it would only be a semester-long relationship . So, my thought is that if they're more than two (or three) years IN SCHOOL older/younger than you, don't even try. I know a lot of you would tell me that's not true, but I guarantee there's an even larger number that can testify to what I said.
Finally, there's my handy rule: when in doubt, use percentages. I prefer to use 75% as a MAX age difference, but 60% (or 50% for lazy children) works OK, too.
Then there's the "maturity" issue. I sorta covered this in the school part, but I gotta add some things to it. Now, there's one main thing about this: maturity counts for a lot, but let's be real here. Maturity isn't gonna make up for a 50% age difference. Also, "maturity" is a very subjective thing.
That's all I gotta say, good thread by the way!