Age itself isn't a good measure. One's maturity level is the better gauge. Of course, that's much harder to measure, so people just fall back to physical age.
But yeah, I've seen people in college with a maturity you'd expect from early high school, and vice versa. Just depends on whether that person is able to make an informed decision on their own.
If you want a specific range..when below the age of 20, probably a 2-3 year range is decent.
Yes, fair point - but it isn't simply maturity. I think life experience is really important.
People of two vastly different ages will find a long lasting relationship difficult to sustain. If you're at two different stages at your life - which is irrespective of maturity - it can be really difficult.
For example, if I were to date a 30 old guy, he may be thinking of settling down, and I wouldn't be ready for that, or to make any kind of commitment. I'm not just talking about romantically, but people my age are very undecided about studying/career, and so everything is up in the air, whereas older people are more 'settled'.
Not to mention younger people have different habits, tend to have faster paced lifestyles and have entirely different priorities and tend to enjoy lack of responsibilities. If I were to say hang out with my hypothetical 30 year old boyfriends' friends, it would be a slightly awkward; I'm not talking about integrating with a few friends, it's the fact that we'd move in different kinds of social circles.
You always feel the need to justify yourself when introducing a boyfriend or girlfriend who is a very different age from you. It will always raise a few eyebrows.
However, I do say this with great reluctance. I truly wish it was fine, but I think there are a lot of complexities which make such a relationship problematic.