Naven
Answers
What is the safest messaging app for a teenager?
There is no single answer, but the properties that matter are consistent: no public directory, no stranger direct messages, screening before delivery, and a block that works without needing anybody's agreement.
Most harm to teenagers online arrives through contact from strangers rather than through content they sought out. A platform where a stranger structurally cannot find or message them removes the largest category before any moderation is involved.
The second property is what happens to the messages that do arrive. Screening before delivery stops a message reaching them; reporting after the fact does not, and the difference is the whole harm.
The third is autonomy. A teenager who cannot block somebody without a parent's involvement will not block them.
Naven is built on all three. It is also run by one person and says so — that is a real limitation worth weighing.