Even moles/birthmarks you've had all your life can turn cancerous.  I think 50% are new moles and 50% are existing ones that change.  That is the key thing, they change if they're cancerous.  It would pay to get your GP to check it.  

Skin cancer is possible in any age. It sounds like your birth mark is just a birth mark, unless it is changing colors or shape, or getting bigger. I couldn't figure out if you meant that you just started looking at skin cancer pictures and your birth mark looked like some of the pictures and that concerned you, or did you notice your birthmark changing lately and THEN you did some research, finding some photo's? You should always "check" birth marks and moles and so on, so that's good you are doing that, you should look for any changes, or if it starts to hurt or itch I believe is another thing to consider. If and when they "change" you can simply ask a doctor to look at it. The best thing to do is see a dermatologist, they usually can tell just by looking if its anything to be concerned about. Is it possible for you to see a doctor? Its scary to have health concerns when you're so young, but you need to tell your parents or guardian or even a teacher that you are concerned. How about a school nurse? They can help more then people on the net who can't see you. I bet you're okay,  don't try to diagnose yourself via the Internet, we can really scare ourselves if we do that. Let us know what happens, K! But don't worry yourself into a panic. Just talk to someone older and show them, K!