How old are the crystal gems really?

obvisly they are way older than the people on earth  and there is proof on the beach episode garnet did say that she was older than the pizza family's grandma but do they ever age? or are they immortal actully this theory doesn't check out they actully explain everything about this theory on the birthday episode they say "we can't age but if we get hurt we can die, but not from food posining" so this theory isn't true :(