I find it amusing how some could get so concerned about Eraqus, Xehanort, and Yen Sid being of significantly different ages. XD Is it considered weird because the age gap between Sora, Riku, and Kairi, or Terra, Aqua, and Ventus, isn't that wide?
It wasn't said that there are minimum and maximum age limits to be taken under a Keyblade Master's tutelage. As said before, Eraqus could've been the youngest of the bunch, like probably 15 when he started training, while Xehanort could've actually been 30 or so. Yen Sid could've been in-between.
In addition to time flowing differently in different worlds, genetics do affect how people age. With some genetic make-up, you stay in your prime appearance longer than others, while with some other, for a shorter period of time. I've seen people in their 60s and 70s appearing younger than they are, as well as those appearing as old or even older (trust me, there are quite a few of them where I live, and they can get truly grumpy sometimes).