Does GNC's futurebiotics hair, skin, and nails vitamins really work, and is it safe for teens, if not is there any vitamins for teens to help me to grow my hair long?

No vitamin pill is going to make your hair grow any longer. However, you could try vitamin B complex. That is good for skin, hair & nails.

I agree. You should also try:

Biotin
Silica
& I heard some good thigs about MSM

Yes they do work, I have hypothyroidism which one of the side effects is slow hair growth ( and I'm talking 1" a year growth). Ive been on GNC's hair, skin and nails for just 3 months and have grown 2" so far, they take a month to kick in but they do work.

Everyone looking for hairgrowth must also use Crudoleum hair treatment prescribed by Edgar Cayce.
However, I think that futurebiotics is a good vitamin that is not strong to strain the system (kidney / liver?) and has stuff like Iodine that was heavily used by Edgar Cayce in his treatments (he used atomidine though)

Be careful not to get overdose of iodine
Be careful not to take the tablets regularly. Give a weeks break every month and rely on raw diet / juices to supplement the vitamins.