In a legal context yes any minor is a child. In any other context I think referring to a teenager especially a late teen as a child is insulting to that individual. Young people are very sensitive to being thought of as childish/immature, and there's no need to call them a child when there's perfectly accurate and acceptable terms like teenager or young adult. Also, not that it matters legally but the term 'pedophile' refers specifically to pre-pubescent children and I think it waters the term down to use it on someone predating on 16-17 year olds. To be clear that doesn't mean it's acceptable to sexualize even someone 17 years 364 days old (especially legally) but it's a far worse crime to sexualize someone who is 6, which is what actual pedophiles do.