John M. Hilinski, M.D.
Thank you for this interesting question.
The earliest age to undergo a nose job is from the age of 17 on. Then, the bones of the face are fully grown. This is for functional surgery (i.e., breathing problems). In Belgium, the legal age for an aesthetic nose job is > 18 year. If you do it before this age, the nose can grow crooked again.
Hope my answer was useful :D