There's an explanation on Bulbapedia that states that "time has not passed since the start of the journey. Ash is eternally 10 years old."
So all the times we saw nighttime or multiple days passing, just an illusion. Ash had gone through Kanto, Orange Islands, Johto, Hoenn, Battle Frontier, Sinnoh, Unova, Decolora Islands, & Kalos ALL in one day. Boy Ash gets around, he obviously didn't miss leg day.
Funny thing is, I'm pretty sure that if you count and add up all the sunsets we've seen until now (usually at the end of episodes), you'd arrive at a number much higher than the number of days in two years. This sort-of implies that certain episodes have occurred in parallel, with the heroes meeting several different characters-of-the-day on the same days, then switching rapidly between them without ever mentioning them to each other.
Not to mention the episodes taking place at night or occur over multiple days. Also ignoring the mentions of Ash's birthday of the anniversaries of Ash first going on his journey and meeting Pikachu.
Then again, who says the Pokemon World has the same amount of days in a year? Maybe a Pokemon World year equal 1000 days. Hmm, actually, thinking about it, have we ever seen a seasonal change in the Pokemon world that wasn't due to them traveling to a warmer/colder location? Maybe the Pokemon World is orbitally locked! It still spins on its axis but it's stuck in the same location in space, thus "days" come and go but no time actually pass! WE SOLVED THE MYSTERY OF THE NEVER-AGING ASH!
"Even if that was true, Ash (and everyone else) would still age".
... time is literally controlled by a diamond quadruped space dinosaur, your argument is invalid.
*Sigh* I don't feel like moving time forward today, maybe tomorrow...