I actually find the rigid attitudes on this thread about what play is age appropriate sort of disturbing.
There's a whole range of paces that preteens develop at, both socially and mentally. There's people here gloating that their kids are so very grown up at 11, 12 and 13, as if that makes their kids superior to kids who aren't quite there yet. Becoming a teenager is hard. Some 13 year olds still want to play like 11 year olds, others think they're 17. It's all within normal ranges.
Dressing up like this allows a nice comfortable middle ground between fashion and play for the kids who aren't feeling "grown up" yet. I was a kid like this, although in my local culture I dealt with it by leaning into a sort of pseudo-hippy aesthetic, all bright colours, psychedelic patterns, would carry a tarot deck around (couldn't even read tarot). For good measure I mixed this in with what I thought of as Native American fashion: feathery necklaces, howling wolf t shirts, you get the idea. There's a very real sense in which they were pretend clothes, like a costume I could put on and sort of play a character in while I adjusted to being a teenager. I outgrew the faze and emerged as a normal person who could choose clothes that I liked and talk to other teenagers normally (well, in truth via the route of becoming a "scene kid", if anyone remembers those). I had a couple of friends like me who were very into anime and who leant into the cat ear/ Japanese kawaii aesthetic. The furry thing is just this generations iteration of that.
By the way, everyone sitting around wondering where kids get therians from - they get it from the wider culture. Media aimed at kids is overwhelmingly about talking animals. Lion King, Kung Fu panda, all the side kicks to all the princesses, all the storybook characters, Pokémon, all this stuff teaches kids - for years - that animals are just people who look a bit different, and that there's something it is like to feel like these animals. Then they hit a specific age and we want them to just stop watching it, or they're "weird" and "being groomed". Don't swamp your kids with talking animals if you have very specific ideas about when they should stop liking the media you've been feeding them for the last 11 years.