[mezzoforte]

I've heard that the younger partner should be at least half the older partner's age + 7. So, an 18-year-old can date a 16-year-old, and a 30-year-old can date a 22-year-old.
Generally, I think that's a decent rule of thumb, though relationships can certainly work outside those limits--my own parents started dating when my mother was 17 and my father 24, and they're still married. Everyone approaches relationships differently, and some young people are remarkably mature. I try not to judge because it's impossible to see inside any relationship, but a particularly large age gap can force a problematic power dynamic.