When a zulu youth matures, the maturing circle stays.
It doesn't do anything else but it can be selected and deleted, but it gives 10xp.
I don't necessarily know if the circle can be attacked by enemies but it just gets in the way. I can only imagine it making a mess once you get a large amount of youths maturing at once.
Also youth count as idle villagers when they are sitting around, it really gets in the way of microing other idle villagers since the game will treat them as idle villagers and select them when your intending to select actual villagers.
Edit: Hmm for some reason they started disappearing now, gonna see if I did something specific that caused it before.
Edit2: Okay it seems like it primarily happens for the villager maturing circle, and it just takes alot longer for the circle to disappear. Not sure if that's intended, since it lets the circle generate more exp than normal.