My daughter is 13.5, and about twice a week she shampoos with Head and Shoulders, and the rest of the week just shampoos and conditions with her normal stuff. She has combination hair so that deep down at the roots it can get oily, but her hair is so thick and long and wavy, the tips can get dry. She was finding that the oiliness near the scalp was causing itching, and the Head and Shoulders really helps with that. Funny thing too was when we went through a bout or two of lice last summer (ugh), the lice combing eliminated all of her dandruff – so now she makes sure to really brush well and often and use a comb at the hairline.
Just on a slightly different note, though, a few years ago, when she first started going through early puberty, I noticed a tan scaliness on her scalp that was causing some itchiness. I noticed it after getting back from the beach and thought all the suntan lotion might have left residue on her scalp (she didn't apply it there obviously, but the spray tends to go everywhere). It almost looked like cradle cap from when she was a baby. I took her to the dermatologist and it was seborrheic dermatitis, mostly on the crown of her head. She said it was a combination of hormones and extra oils coming in near her roots. For that she had to use a special prescription shampoo, and the tan scaling came right off and never returned. We only used it for a little while, but it really worked.
So you may want to check it out – regular dandruff, go with option 1 above and the tan stuff go with option 2.
