We have age discrimination here in FL all over the state. 55+ communities do not allow anyone under that age to own property there - private community. Sorry, it is still their house their rules. Just like a restaurant that says no shoes, no shirt, no service, C-stores that say no hoodies or no more than 3 students at a time, night clubs that do not allow single men in, and on and on. It happens all over the place, it is their right, especially as a private organization to allow or disallow whatever and whomever they want. Not a Catholic? Good luck joining something like the KoC.
It is simple, either ask them to change or go somewhere else where your views and actions are wanted/appreciated.
I wouldn't want to force my way into an organization via threat of lawsuit or whatever - what type of friends do you think you will make?