To the 14 year old contemplating whitening her teeth.   If you were to ask
my opinion, not a dentist, a mother, I would say, if it aint broke, dont fix
it.
The goal is to have teeth  to be as strong as they can and whitening can do
nothing to strengthen them.   Why risk anything uncalled for except for a
little vanity in this particular
manner when existing conditions are not really that unsightly
Gail Michael