He's officially always ten! It's comic book time, like how Batman/Bruce Wayne (unless for a special past or future story arc or alternate take like The Batman's old Bruce) is an adult or Peter Parker's always in high school.
The way he's played in the Japanese version, Matsumoto Rica clearly deepens his voice and lets him sound older, like he's going through puberty—compare his high, youthful Kanto voice with even Battle Frontier (late Advanced Generation) or Diamond & Pearl!
Sun & Moon's portrayal is so different, she prefers thinking of him as having known only Kanto before coming to Alola, so she plays him more youthfully and excitedly to match the usual tone of this more slice-of-life/shool-based series.
But, she also jokingly calls him "the eternal ten year old" because of how he's officially always ten.
You're free to headcanon him aging, Matsumoto-san likes to headcanon too, but officially, Pokemon is a timeless work, it operates in comic book time.