@Platus

More interestingly, how does this fit with the theories of alicorn aging?

After all, she was an adolescent when Twilight was a filly, which would be odd for her to only progress that far in 1000 years and then suddenly start growing at a normal rate with Twilight.

Perhaps she's a descendant rather than the original princess?

Ah, that explains the chrystal heart on her flank. So what has she been doing for the last 1,000 years if she was the princess the whole time?