What age did your son start "messing" with himself?

Me and my friend found her 7 year old autistic son with VERY VERY DISTUBINGLY GRAPHIC MAGAZINES under his bed. He was playing in his room and he went quiet so we checked on him and he was under the bed looking at the pictures with his hand down his pants masturbating. We were horrified, we took the pictures away, he went in the corner. She doesn't know where to begin. Help?