As I tend to play historically-themed games, I spend a fair bit of time "kidnapping" NPCs on comm lots (using "Gussy Up" to get them a new, period-correct wardrobe). Yesterday, on a residential lot, a townie child wandered through, and when I went to do the gussy up thing (buy clothes) I noticed that both the age and gender were incorrect in the clothing catalog dialog that popped up... it was a CF sim, but it was the TM clothes that were showing up. So, I changed it to purchase CF clothing, and that's when I noticed, during the "jump and spin" changing animation, that the kid's feet were not touching the ground.

Today, I had just moved in some folks to a new lot, and when one of the neighbors showed up with her kids, I saw that they, too, were levitating. These were sims that I had created, whereas in the previous case, it was a townie. I haven't been to their home lot to see if they were indeed the wrong age and/or gender, but I imagine they probably are. I'm afraid the neighborhood is becoming corrupted. Can I repair things with the Sim Blender, or should I do something else?

When I saw this I immediately thought of their Sim Description being wiped.
Sims that don't have a Sim Description revert to a default one:
Their age stage is always Adult, they are always male, no personality, fitness maxed without the matching symbol, and no aspiration (The level is always "Cold Fish", set to the default romance aspiration failure title.)
What I don't understand is that the townie was set to teen male clothes. It usually gets set to AM, that's the default Sim Description.
The kids were levitating because they were set to TM. (I'd assume because of the TM clothes. In normal situations they would be set to AM.)
Do their animations look weird, setting aside the levitation?
As with the other kids, I assume they have the same issue.
The townie child being broken reminds me of Elle Rodiek and Jeannie Chen, two CF townies that have their Sim Description broken in Bluewater Village. But they are not in the townie pool and thus can't be a walk-by or appear at community lots. I'm saying this to compare the two cases.
Can you go to their home lot to check? If they have the same problem with the townie child, then we can see what the issue is.
Also check with SimPE.

If I don't remember completely wrong, do you have the Mac version of the game? If so, could you be nearing the file limit? That could (I think?) cause issues like this, where they either aren't saved properly or are saved with the wrong information.

Yes, I have a Mac, and that could very well be. I altered the system code to jack up my file limit to three times the original, but... that was a while back now. It's starting to act weird, where it won't let me build lots unless I do a test build first (like just one wall before saving), and quitting out of CAS if I try to enter a family name before I've built any sims. So it's probably time for a CC purge. I've temporarily abandoned my non-historical hoods anyway, so maybe I'll put all the relevant CC into storage for now.

I've had my own Sims turn up as the wrong age before. The townie child appearing as the wrong age was a weird one, though... maybe I had previously hijacked her in a comm lot to Blender-change something about her, although she was wearing modern clothes, so I don't think that was the case. Anyhoo... on we go.

I will go to the family's home lot in a bit, to check on them. I've had problems with that family before, so it wouldn't surprise me. I actually have two different families living on that lot, a total of nine sims, so it's not always immediately obvious whether an issue is a glitch, or just the consequences of crowded living.

The feet animations did look a trifle odd, but nothing drastic, so I might be imagining it. I tend to not play with shadows or reflections turned on, so maybe things look weird most of the time. And... I'm a Mac user, so the usual straightforward troubleshooting steps won't work.

I've previously been messing around with "aged-up" kids/toddlers before (for picture-taking purposes), and when changing clothes with a dresser or simple wardrobe-type mod, it showed the correct age for their looks (child or toddler) and not their actual age (teen/adult). Most of the time I'd already bought clothes for them, so I don't know what the "buy clothes" menu would show, though. It's possible the "Gussy up" menu in this case acts more like the buy menu, or that it pulls the actual age.

Levitating kids/toddlers is usually a sign of them acting like older sims, especially if they're able to do stuff younger sims aren't normally able to do. I think teens use a slightly modified adult skeleton for animations, so the difference between a teen and adult isn't very visible (perhaps a bit of hovering, plus some slightly out of touch animations because of shorter height/limbs, but nowhere nearly as bad as child/toddler would be).

Jeepers, were there a lot of problems. Every Sim in town is an adult, although they don't all look like it. Fortunately, most of my sims (in this hood anyway) are supposed to be adults; there are only a handful of kids, teens and elders to change back to their real ages, and since they never left their correct body types, I at least don't have to worry about re-doing their faces or anything. The lot in question with its nine residents (five of them teen or younger) was the worst one to deal with. I tried deleting Sims to see if that would reset them back to their proper ages, but it didn't. Yeah, I have to re-set all their outfits, but those are still owned by the families, at least, so it's not TOO terrible. Some females have become males, some not. I have that "personality adjuster" from Insimenator or wherever, so that's not a big deal... couldn't figure out a way to do it on the blender. A few aspirations had become Grilled Cheese, or otherwise garbled; many relationships were blanked. I had to re-set a few marriages. Everyone was also unemployed, a little annoying, but I fixed that pretty quick. After I get my elder-ified black toga recolor from my recoloring guru, NO MORE CC, for a while anyway. I did go and remove a bunch of stuff I'm not currently using, and could probably stand to remove some more. We'll see if things improve.

Yep, I think I understand now. Simmer22 is right, you're probably reaching the Mac file limit. That's why they're changing to adults. It's weird though, it's like their Sim Descriptions are being half-wiped instead of the usual reverting to the default one. It's like on some Sims it wipes different information. On some it's aspirations and on others it's gender. I think that proves you're reaching the file limit.
It's a good thing you can fix most things. I hope nothing more gets broken by this.