I don't think there's a clear-cut answer for this. Personally, I would say two years is an acceptable age range in the teen years. An individual changes a great deal between the time they become a teenager and the time they enter their twenties, so if there's more than two years' difference some of those changes can cause a significant rift in relationships. I'm not saying that's how it is for everybody, but I've seen it happen a couple of times.

Now, if sexuality is involved, then you enter matters of consent. I'm from the United States, and in the US an individual is considered a legal adult at the age of eighteen. Different states have different laws for the age of consent so it varies, sometimes in general and sometimes based on whether the relationship was preexisting when one of the individuals turned eighteen. However, that's a whole other topic.