Yes, its normal to have wet dreams at any age, and frequency like that. It is very rare though. Have you always had wet dreams this often? Or did they just recently start happening?
I love wet dreams, and only had one in my lifetime (at 19). I wish I could have your 'problem'.
You could try increasing your masturbation frequency to once per day everyday, and see if that reduces your wet dreams.
And to answer your second question, no, wet dreams are not always caused by sexual dreams. Many times guys have wet dreams, ejaculate in their sleep, and wake up not remembering any dream at all. But sometimes a wet dream can be part of a very exciting and intense sexual dream. Both are normal.