A lot of people have through the years made many remarks about the Pokemon characters and how they have barely aged or changed externally in any way since the series began. However I want to provide an alternate perspective on this. First of all let's look at another anime that had many episodes, Sailor Moon.

SM consisted of two hundred episodes and three movies (plus several specials). During this time they faced six different sets of villains, died and were resurrected twice, and saved the Earth, their glaxy, and in the final season the universe from being taken over, destroyed, or reduced to nothingness. During all this a mere two years passed in their reality.

My point is that a lot can happen in a short amount of time from our perspective, with individual episodes being perhaps a day apart from the other in the character's reality. Thus the lack of aging of the characters might be able to be accounted for and reasoned by this timeline, giving what the series does say about how much time has passed.

The first season apparently lasted one year, plus two months (I think) until the Pokemon League. We are given no more info on the passage of time for quite a while but I think it's reasonable to speculate that the whole Orange Islands saga could have lasted only three months or less. Johto used far more filler episodes than the other leagues, thus the amount of time that passed may have been quite a bit less than what it seems to be. Its possible it lasted little more than a year. Then there is another confirmed word on time when someone tells Ash it will be X months (can't remember how many) until the Johto League Tournament starts.

And so if you want to be conservative its possible to see about three years having passed since Ash began his journey, enough to account for the limited change in the characters appearance's.

Then there's the fact that Ash and company could simply be naturally short.

Or maybe I'm just looking into this too much...