If it's real expensive, insurance won't want to pay.
Probably have to be 4"2' at the age of 16 or something.
On the other hand, an actual, old fashioned, private doctor (do they have those any more?) might prescribe it if you just ask him real nice.
I'm interested in this question, and also, how do you find a doctor that doesn't use insurance (I'm pretty sure rich people have them)? or is that just totally out of my league.