From what we saw on the glass window, Flurry Heart clearly aged up in those 20 years. And if the cannon chapter book is anything to go by, Cadance aged up regularly despite becoming an alicorn at a young age.
Just that; how long is a year?
We know that everyone ages - even alicorns - but as Luna had shown, one goes through several phases from the foal-like state that when she came out of the shell after Nightmare, it appears that there are at least two or three different “forms” of an alicorn. Twilight Sparkle was in “foal” condition when she got the wings and become an alicorn. This suggests that age is not just related to time, especially to those with very extraordinarily strong magical potency. There are only three alicorns that reach what we might call the “Queen” mode - Celestia, Nightmare and Twilight.
We do not know what happened to Cadance, but her role as governing monarch in the Crystal Empire was obviously not politically stable as she was seated on the throne of Celestia after King Sombra was removed, which is why her daughter born in the Empire, Flurry Heart, was so important to the citizens - she had become their true ruler as “one of them”. Then it is natural for her to retire when her daughter was old enough to take over. It is obvious that there is a story behind the glass window.
The alicorns we don’t know much about, only that they are among the strongest magical beings in the known universe, surpassed only by a few like Discord, who had distinguished themselves by being very different in the magical field. The alicorns are at the top of “the magical food chain”. And we know the magic of Equestria is not always logical.