You wrote (and I quote)
on a legal advice website, and then take offense when that statement is interpreted as "please help me sue"? Really? I, too, am starting to suspect the lack of interest in you volunteering has nothing to due with your perceived age.
But do let me reiterate what already has been said, multiple times, here already - the school does not "have" to let you do anything; you can't force them, legally or otherwise, and even if their decision was based 100% on you "looking young", then that does not make a shred of difference to either of the prior two answers. (And just so we are explicitly clear, I am a licensed attorney who has been handling all sorts of discrimination claims for just about a decade now).
Is that better?