I never see it discussed, but in Pokemon 4Ever; Ash and Pikachu swim in the Lake of Life, which is essentially a fountain of youth. Misty dips her legs in, and Team Rocket falls in at one point. Brock was also there, but we never see him go in.
While this isn’t established as an official explanation, I choose to believe that’s why none of them ever age.
I never see it discussed, but in Pokemon 4Ever; Ash and Pikachu swim in the Lake of Life, which is essentially a fountain of youth. Misty dips her legs in, and Team Rocket falls in at one point. Brock was also there, but we never see him go in.
While this isn’t established as an official explanation, I choose to believe that’s why none of them ever age.