traveller tbh, it's just because (especially in jcink communities because of the advent of jcink premium, i think minor characters are generally allowed more often-- but not always-- on pb sites) when writing communities grow older, a lot of people like to tackle more mature themes and topics in their writing. the large majority of people are the most comfortable playing these concepts out through older (18+, but i've been seeing a lot of 21+ character bases now, too) characters. even a pokemon rp can be set up in a way where the structure is catered towards adult characters, despite the source material, in the same way i've seen it done w/ fanbases like atla & harry potter.

as a result, while some sites will ban child characters partially under the assumption that people might be bad actors when playing child characters in an overall darker setting, i think a lot more sites ban them just because it doesn't really align with the character base; in a roleplay that primarily is set up for and caters to adult characters, it's going to be much harder to involve child characters in plots and in group events. it's a practical 'hey, though there are kids in our setting in npc roles, we don't think a twelve year-old is going to fit well as a player character here'. even on slice-of-life irl fc sites, where i think 'no kid pcs' rules are way more common than they are in animanga spheres, that aren't any 'darker' than, let's say, an episode of abbott elementary, playing kids is banned largely because the character base is largely made up of characters in their mid-30s, and a kid character has the chance to stick out like a sore thumb when you're trying to meaningfully plot with other writers.

i think it's silly (not every character that appears difficult to plot with will be and it's silly to make that decision for a member before they can even try it out, i played with a delightful old grandma character a couple years ago that i don't think you'd really see around these days), and i also think people making bad-faith assumptions about anyone who rps kids is ridiculous, but the forum rpc is aging and i think that just means you're gonna see a lot of characters age along with that. i've already seen character and member age limits bumped up from 18+ to 21+ to 25+ within the forum rpc.

honestly i'd recommend looking for pokemon sites/discords/etc. that are all-ages if you're looking to play a child character or coming-of-age story, since i think sites with playerbases including minors themselves are more likely to include all ages of pcs! i've seen a couple pokemon boards sites advertised around recently that are all-ages both memberwise and characterwise that might reflect that classic pokemon childhood adventure a little better-- you might just not be able to find them in the same places you used to.