Because if you were in porn and you're under the age of eighteen it's considered child pornography, due to the fact that you're not considered legally an adult yet.
Technically, you're not meant to send naked photographs of yourself to your boy/girlfriend or whatever as that's considered child porn too and then you can get done for the production of it, but in honesty nobody really cares much about that part because it's hard to find out about.
If you think stuff like that is weird, in most parts of America you can't drink until you are 21 but you can have a gun licence at 18!