Is it normal for a teenager to wear diapers?

It's not normal for teenagers to wear diapers, unless they have some medical condition that inhibits their control of their bodily functions.

Teenagers are expected to be fully toilet trained.

For someone who've suddenly developed a need for diapers there are medical issues that would need to be looked at with some urgency if it is happening.

An appointment with a urologist is called for immediately.

A small number of people like to wear diapers as part of (sexual) role-playing, but that's quite rare.