Could you give us some more examples than just Pokemon/Digimon?
Because I've browsed through some anime sites, but I couldn't really find any...
Here's my theory about the 'young designs':
The protagonists of Pokemon and Digimon were/are around the age of 10 for a reason; because they were/are meant for CHILDREN (around the same age as the protagonists) to watch. And Children around 10 years old are satisfied quickly. I assume that drawing 'old' people is a little more work than drawing kids; The people who made the anime thought: 'why spend more time on drawing old people, if the viewers already be satisfied with just kids as characters in the anime'. And that's why some anime contain only kids/young people in it
