Is It Good for a 15 Years Old Girl to Whiten Her Teeth?

Yes, but only with over the vounter strips right now. It is also best she does it under doctor supervision.

I would stay away from over the counter whitening products for now.  At our office we have whitened child actors teeth as young as 10 yo with great results.  So as long as you whiten at the dental office, I see no reason a healthy 15 yo shouldn't whiten their teeth.  (Make sure she doesn't have any filling in her front teeth, they will not whiten and will look yellow after treatment).  Teeth whitening makes a great sweet 16 gift and a very strong motivator to keep up good oral hygiene.

Over the counter products are unpredictable and may be harmful.  Under supervision of a dentist it should be fine.  I whitened all 3 of my daughters teeth, as early as 10 years old.  This was completely monitored and controlled by me with nice results and no danger.

Not a good idea at this time.  The tooth nerves are quite large (they shrink back with age), and the teeth may become very sensitive.  Wait at least to age 18