I'd send them a BUNCH of links from several locations about when knees close, when backs are 'grown' (lol), when horses finish growing, age ranges when a horse can be ridden (I'll even accept 2 year olds, but yearling - NO), etc. Just flood her with all sorts of articles which say WHY you don't ride a horse before two (and better to wait until 3 or 4!)

An eye for an eye way of life may make the world go blind.

Maybe then we would learn to start listening.