Probably at least 3 years old, but not too old to be easily trained to ride.
I'd probably train it to do just general riding, and take it on trails. (probably using some CA methods, and getting some help from my trainer and parents)
I just feel like it would be a nice thing to do instead of buying a horse from a breeder... What are things to look for in a rescue horse? Do any of you have rescued horses/ponies?