I think it'd be smarter to stay in your age group because so much of adolescence is based on what your peers do. And if you have a boyfriend that's 17-18 and you're quite younger(let's say 13 or 14) chances are, they'd want to do more than what you'd be comfortable with ;P soooo, I'd say 1 to 2 years difference.
Edit: But then again, most 16 year olds are comfortable with almost any sexual act.. So it'd still only be a 1 year gap else it would be illegal.. I guess.... But 13 on 17 would seem odd to answer that girl.