There are three years between the Kanto/Hoenn stories and the Johto/Sinnoh stories, an indeterminate number of years between the Johto/Sinnoh stories and BW (though probably at least five - it has to be at least long enough for Caitlin to age a few years and for the Rocket grunt who stole the Machine Part to consider reviving Team Rocket, decide against it, return to Unova, find a woman who he will get married to, and have a child with her who has the Schoolkid sprite by the time of the BW story), two years between BW and B2W2/XY, and then two more years between B2W2/XY and SM.

So Wally was 10 in ORAS, and then by HGSS/Platinum he'd be 13, and then 17 when you factor in the four years between BW and his appearance in SM, and then you go back and add however many years you like to put between HGSS/Platinum and BW.

So yes, 20+ is probably accurate (30 is probably way off, though).

He doesn't "look" "older," but neither do a lot of young adult actors these days. Look at Thomas Brodie-Sangster, for instance.

On top of that, we've had a fair number of people in the Pokémon universe who don't quite look their age. Lusamine is 40, Leafeon User Linnea takes care of her skin to the point that she still uses the Swimmer model in comparison to all of her gray-haired compatriots, and then in XY, there's a Rising Star in the Battle Chateau who says that she's actually 39.

Most of all though, they probably just didn't bother to update his model since they'd just created a perfectly usable one for the previous games.

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