Cadence was visibly younger while watching filly Twi, which was before Episode 1. That means that she ages normally, and that she was likely born while Luna was on the moon.

See: Probably not Luna's daughter.

Well, that would make Cadance over a thousand years old for one. It would also make Luna the biggest bitch mom ever for missing her daughter's wedding and really not seeming to give much of a fuck when she did eventually show up.

Would have a bit more credibility to say she is descended from a line that Luna started before her banishment. Hmm, I wonder what it would be like to leave a young son or daughter behind and come back to meet your great great great great however many greats granddaughter.