Young people can need privacy most when they are least practiced at asking for it.

Privacy is a skill before it is a right, and the skill takes years to acquire. A young person working out who they are needs somewhere unobserved to do it, and simultaneously has no fluent way to request that without it sounding like secrecy or accusation. So it comes out as a closed door, a short answer, a sudden change in what gets shared. The bluntness is inexperience, not hostility.

If you are close to someone at that stage, offer the space before it has to be demanded. Say plainly that not everything has to be reported, and then leave the room.