Can a teenager have plastic surgery? (Relax, I have no intensions; just curious, is all)

No, because your face is still growing and if tu get it at that age, it'll definitely fuck up your face. But if teenagers could, damn, I'd be the first in line.

Yes.

I don't know really..

Yes

Technically, yes.

Technically - they can, but only with a doctors permission and with a parent to sign the legal forms. However any doctor/surgeon will be very wary of doing any cosmetic surgery on teenagers are they are still growing, most will wait until the correct age of 18 just to make sure.
However some surgeons will give cosmetic surgery and fit it around the growth stages if that makes sense.

If they are under 18 they can, but their parents have to agree sign papers & stuff, though.

If they are over 18, they can do whatever they want as long ad they got money