A teenager needs privacy that does not have to be defended as proof of innocence.

Privacy that has to be justified is not privacy; it is a suspended accusation. A young person made to explain a closed door, a locked phone or an unshared conversation learns that trust depends on constant proof, and the usual result is better hiding rather than more openness. Room to be unobserved is part of how someone becomes a person.

Grant some space by default and say it is not conditional on good behavior. Ask about their life without auditing it. Keep serious questions for genuine concerns, and say when something is one, so the difference stays visible to you both.