I'm not quite sure if they really don't age; it might just be more like they age really slowly. One theory I read is that Rose's "death" caused the other gems to be forced to grow up/mature, so to speak.

One theory I had was that sometime between Greg and Rose meeting and Steven's birth, the three other Gems had to retreat into their gems, resulting in the appearance changes. It would make sense, since both Pearl and Garnet now have different clothes after they reformed.