I'm not entirely sold that evidence of them reversing in age is evidence that theyve aged- for all anyone knows their appearance could be a representation of how much power they currently hold. If Luna was stripped of some power, or partially sealed, then that would explain her regression in appearance of age. Perhaps I'm assuming too much, but when I think of Celestia and Luna, I have the impression that they are nearly the same age- just Luna was born second. Like twins, so to speak. If they had dual responsibility to raise the sun and moon, wouldn't they both have to be relatively the same age to start trading cycles? I suppose its possible if Luna and Celestia inherited their abilities. Perhaps they stop aging after they reach a certain point. Or, technically, they could just be using different healing spells on themselves to regenerate parts of their body at a time, and thus remain immortal, yet normal. I think that Celestia and Luna are very close in age, however, I do not believe they are twins, nor were they born in the same year. I do think they were born close together, within the span of a few years, but not at the same time.