It will still benefit your body. There are essential vitamins and minerals that help with a person's skin, hair, nails, and metabolisms. I have heard of men taking them to help with skin blemishes. One thing is though, a woman taking prenatal vitamins has a increased chance of getting pregnant do to the increase of folic acid due to taking the pills.
NOTHING REALLY! You may grow stronger hair and nails, but if your under age I recommend you using just a multivitamin. Like I said if you just want stronger hair and nails and your under age use just a multivitamin that is tailored to your gender and age.