Hey there guys. My roommate and I were talking about it and I think we have come up with a plausible reason why Ash has remained 10 years old.

It is because of the size of the planet and its rotation around their sun. Let me explain.

Because of the several regions that have been introduced over the years, the geography of their planet has always been a toss up.

Using our own continents as examples, seeing as how the show was made here, I have figured the square mileage as follows.

Average Earth Continent Square Miles:
9.5million

Total Earth Continent Square Miles:
57,308,738

Total Earth Surface Square Miles:
197,000,000

There are 16 confirmed regions within the poke verse, along with several small non affiliated kingdoms and states introduced through the show.

Figuring that each region is a rather large country unto itself, it would be safe to assume that 2 could be on each continent. That is of course including those smaller places that we don't get to see a lot of in any particular episode to fill in whatever land gaps that separate the regions.
My reasoning:
Each region a large country.
Small areas that we don't get to see the extent of.
There is always an ocean surrounding most of the game maps. Meaning each one is either a continent themselves or takes up a large part of it.
There is always the chance they will introduce new regions.

This gives us 8 continents.

Total PokePlanet Continent Square Miles:
76,000,000

That equates to a 30%+ increase over Earth.

If we translate that to their oceans as well, we get this.

Total Surface Area of PokePlanet (Including Oceans):
262,000,000

That is a 65million Square Mile difference.

Their planet is much larger than ours. Meaning of course, more gravity! And gravity as well all know thanks to our good friends Einstein and Hawking, stretches time and makes it "slow down".

That is factor #1.