I've noticed there's a lot of entries on TV Tropes regarding the differences between AN AMERICAN TAIL and FIEVEL GOES WEST, noting that Tanya seems significantly older in FIEVEL GOES WEST, but that Fievel, and even Yasha apparently haven't aged hardly at all since the original.
While, yes, it's evident that Tanya hit quite a growth spurt between the two movies, and yes, she suddenly seems older than Fievel, as opposed to them being around the same age in the first movie, it's clear to me that neither Fievel or Yasha were immune to aging between movies: Fievel certainly seems a bit older and a little more mature in the sequel than he does the first movie; as for Yasha, she's an infant, almost always bundled or swaddled in the first movie, but in the sequel, while still a baby, she's at least able to move, crawl, and even walk a little.