>>37030176>>37030401Alola is only set about ten years after Gen 3 which takes place at the same time as Gen 1 and these tell you that Red and Green were at the league years before meaning she's at most 10 and probably 8 years younger and was up to 19 to begin with.
As for Wally, he was said to be sickly which would tend to stunt growth and it's not like we have any idea how old he was to begin with. For all we know this dude's 20 in Gen 3 and just doesn't look it. People don't always look their age. Especially in anime shit.
My younger brother is 29. He looks 12. If I were to shave I'd look more like 20 and I'm in my 30s. Also, pic related is an anime grandmother and time and again you see loli 100+ year olds in anime.
What someone looks like is meaningless in telling their age to begin with and animation just makes it worse.