Oh, I remembered that kinda wrong for a moment, but I was talking about that short before the third movie (The Pichu one: https://bulbapedia.bulbagarden.net/wiki/PK07 ). It wasn't a birthday, it was a party to celebrate one year since Satoshi and Pikachu met. That would imply that he has had a birthday since, that should not be possible without him turning 11. So, that movie Satoshi would already have been 11 back then.

Meanwhile, for his age to make sense at all, in the main timeline, the Pokemon world would have to have very long years. Many thousands of days long, realistically, probably over 10 times longer than in our world (I mean, each episode tends to be on a separate day, and then presumably there are multiple days of travel in between, so 10 times longer is actually a very conservative episode, for over 1000 episodes being only a year). So, when Satoshi began his journey, he HAS to EASILY have lived the equivalent of like, 100 real world years. I guess that's possible, considering how stupid people in the Pokemon world are. Makes sense for it to take a real world century for them to barely know how to do basic addition (subtraction may be too much, he may need 20 more years to figure that out).

Also, yeah, I heard good things about Sun & Moon, about how the story was better than before and how the animation got a lot better, but it annoyed me how he looks EVEN YOUNGER in that. Looks nice overall, though. It has good female character designs.