I personally think credit cards are a great thing for a teenager if used properly. Right now I have four. Two I use to pay for food/other small purchases, one I use as my card for any trip expenses, and one I rarely use (don't even keep it in my wallet) as a backup.
The trick is, like others have said, is just make sure you can pay them off every month. I usually charge about $100-200 on each of my two main cards, so I am sure I can pay them off.
You do need open lines of credit to build up your credit when it comes time to buy a car/house/other major purchases (I just bought a brand new Jetta in March and had the option of financing on my own if I wanted to...but it was cheaper if I co-signed with my parents), and getting a few credit cards that you use sparingly is a great way to do that.