I totally agree with trillian
I would say the same to my own dd of 14 if she wanted to date a 17yo
why would a 17yo be interestd in a 13yo (apart from the obvious), this is a very large age gap at this time, why doesn't he have a gf of his own age ?
when my dd was 13, there was a 16yo boy rather pestering her on MSN, phone etc
he was a "problem" boy who had been in trouble, rather aggressive, known for hanging round with younger kids and virtually shunned by his own age group
rather sad for him, but no way was my dd going to be drawn into something
we made her block him on MSN and a word was had with him by my DH to stop calling. We tried to explain why we could not condone it, she really didn't understand our concern, said "he was always alright with her" etc, caused lots of arguments but she was still very much below the age of being able to make an adult decision IMO
it fizzled out after a few weeks
he tried to show his face again recently, but was warned off again