Should a 16-year-old use a regular doctor or a pediatrician?

Pediatrician: pediatricians are "regular doctors" who routinely see children through age 21 years old. Some doctors will transition a child to an adult doctor at 18 years old. But there should be no reason why a 16 year old can't still be treated by their pediatrician.

Ask the kid: I routinely saw kids up to 18-20 and many had trouble transitioning to another doc when I informed them it was time to move on. That said, those with little interest in their pediatrician would be ready to transition to a family doc or internist.