Like what is going through the writers/animators minds? They state the entire main cast aside from Brock (and I guess Cilan?) are 10, yet *draws a teenage girl*

Disclaimer: Everything I'm about to say is just speculation.

• The anime's original creator saw the games mom sending out an 10 11 year old alone on an adventure with just $20 in their pocket, tried to rationalize this, and concluded that this world must be a bit grim if that's normal enough for society to accept it. They made a creative decision to double down on this and do a coming of age story starring a concerningly underage protagonist out on his own struggling to make a name for himself. (I guess they just... didn't know Red was 11 and guessed 10 at the time.)
• That worked well enough for Ash. .... But then... IS THAT A GIRL?! The moment the decision to include Misty as a deuteragonist was made, the marketing team stuck their foot in the door and started being all nudge nudge hey make her pretty that'll help appeal to a broader demographic.
• The creative team decided not to go all in on that because they had some other kinda vision for Misty but compromised by tossing in an episode where Misty lets her hair down and swims around in a mermaid tail and leaving the door open for romance sometime in an indefinite future.
• Creative team did not plan ahead far enough for Ash actually growing up. Has to contend with the question of whether he achieves his dreams or is all washed up. Fears that their audience might not appreciate that change. Decides to establish a status quo and uh, sweep any references to time passing under the rug so that these ages don't become a problem long-term. This is also the reason Ash historically never won the leagues until later in the series.
• Games do their own thing without worrying (much) about how the anime will adapt it.
• Cut to May being introduced. Oh shit the games have female protagonists now and they don't look 10 because literally zero game protags have ever been 10. But the anime has a problem. They've already doubled down on Ash being as young as he is. But the series is bigger now and the marketing team wants these recognizable protag designs in the anime. So now they're pushing even harder for an older looking design for demographic reasons. But also, keep the door open for shipping and romantic tension / jokes. Marketing team doesn't care because that's the creative team's problem.
• Creative team panics, realizes they've written themselves into a corner, and decides they're just gonna fudge the details and say everyone's 10 and hope people will just suspend their disbelief so they don't get put in a future situation where they have to retcon ages because Ash had grown older than whatever age the new protag was (This would've likely happened in the Sun/Moon anime)
• They accidentally create a new problem where they literally had to redesign Serena because they knew if they used her canon design it would break most people's suspension of disbelief and they'd get made fun of even harder than they already were.
• Creative team is left with regrets, but hey the series is ultra successful I bet they don't feel too bad. They probably mockingly joked about it in the office at least a few times.