I think May's age and level of development are a central aspect off her character, even in the games. I also think the general high quality of her character (being a much better insert for players than Brendan) is why adding female protags to Pokemon was less controversial than that change in some other franchises. And I know Crystal did it first, but that was a re-release/definitive release. I never played Gen 3 when it came out, didn't play anything between Gen 1 and Gen 5, and going back and replaying all of them, May is tied with Red for me as the best player character in the entire series. She's just the perfect kid for a coming of age story. Her whole design is based around this. She's an elementary school girl because that's the audience they're trying to rope in, having already got the boys with Gen1/2. Compared to other female protags (and Brendan) she's very competent, capable, and non-feminine. Just like you can imagine Red carrying all his items around in his giant pockets, she's got a pack around her waist. She's prepared. Brendan has no such bag and a perpetual stupid look on his face. So May is more like Red than Brendan, but they didn't want her to be a total tomboy, hence the overdeveloped feminine figure. Since there's no character dialogue, most of this is conveyed by things other characters say, but everyone May meets is impressed by her skill as a pokemon trainer. She's constantly prying adults out of horrible jams, and it doesn't feel out of place because, just like Red has that hyper-competent cocky look on his face, May just LOOKS like she's older and more confident than her age would imply, and it just fits the story telling in RSE so well.
May is one of the only female characters I have no difficulty self-inserting as. I couldn't self-insert as Brendan if my life depended on it. To me, she's the default RSE protagonist. I wouldn't change anything about her. Perfect execution of what they were trying to do.
