>>41752343Guys it's fake... The twitter doesn't exist, and we have confirmation the art book doesn't mention characters' ages.
Game Freak hasn't been giving concrete ages to these characters since Sun and Moon, it wouldn't be a surprise if they continued to with the Gym leaders & rivals, especially with Marnie since she becomes a gym leader.
Not to mention your character is even more customizable this time around, so your trainer's age is really up to your choice. Makes sense your rivals would follow suit.