Yes, indeed---who set this age limit?
My guess it's some people who never learned to throw it, and they think no one else should either. The fact is that the slider, when you throw it correctly, is easier on the arm and shoulder than almost any other pitch. This was told to me by the pitcher (an active major leaguer who used it a lot) who taught me to throw it. No abstruse technicalities, no complicated expositions. He simply told me, "Throw it like a curve, but roll your wrist, don't snap it." He showed me the off-center grip---and this is important, the grip is indeed off-center---demonstrated the wrist action, which once you get the hang of it is a lot easier---and then handed me the ball he had with him and said, "Go ahead, try it." I did, and in about ten minutes I got the idea.
I worked with it over the winter, and it became my strikeout pitch. I was sixteen at the time, and I don't think that this is too young an age to try it. Now that I think of it, I think that the key to the whole thing is relaxation. Also, I threw sidearm---I threw all my stuff that way---and I never had any arm or shoulder problems with anything.
So if you want to try that pitch, work with it and see what you can do with it, go ahead. 8)