It's worth noting, 17 year olds would normally have to SEEK OUT opportunities to even be around 13 year olds. It's creepy for a 17 year old to date a 13 year old because their very meeting is questionable. If, however, they work together as peers at an after school program it is a very different situation than someone stalking younger kids in the hallways. I know I didn't know any 17 year olds when I was 13, nor did I know any 13 year olds when I was 17, so this situation would never even come up. The only reason this seems weird is because we segregate kids by age group and they almost never intermingle. I would agree with you if this kid actually went out looking for a 13 year old to exploit, but based on OP's post and her responses, it's sounds like he's not a stranger at all.

Going to college, buying cigarettes, joining the army, having a driver's lisence are just meaningless milestones. I've never bought a cigarette nor joined the army - does that mean I'm not an adult? Of course not. So why would having the mere capacity to do these things imply that someone is. There are plenty of 17 year olds out there drooling over celebrities and being kids. You're not wrong about brain wiring developing rapidly, but that doesn't necessarily mean much for maturity - since everyone is different and everyone grows at a different rate. You can't just standardize the age at which children mature and expect everyone to fall nicely into categories.

Regarding legality, that really depends on the location. I know here in canada age of consent is 16, but there are romeo and juliet laws for people within 5 years of age of one another. So no, odds are it is very much not illegal unless you live in Connecticut where those laws only allow relationships for people within 3 years of each other.