The only thing I have to say is that the sample size is really too small to tell anything definitively . We don’t know if Luna is really immortal like Celestia (She was locked into the moon for a thousand years, but may not have been active so didn’t really age, like Fry from Futurama when he got out of the cryro lab. I’m not saying she isn’t immortal like her sister, but there is no 100% certainty there). We only know four Alicorns, and one of them was only through a book and may or may not still be alive. I really don’t think the show will explicitly say that Cadance can really live as long as Celestia.
TL;DR: We really don’t know enough to say that this is right or wrong, and I doubt the show will really try to explore this for obvious reasons

Perhaps, assuming that Celestia and Luna are equivalent to deities, then Alicorns like Candance act as demi-gods, having the physical characteristics of gods but still being mortal. We already know that Celestia and Luna are immortal, but Cadance seems to age like anyone else, so we can probably assume that it's something similar.