Hi Savyjoy!
One of the main reasons why you should curb this habit is due to the mess (poop) they cause but also in some breeds its not good for there legs to sleep like this. There are a few reasons as to why your chickens could be doing this;
1. Nesting Box Height make sure you nesting box isn't higher than the roosting bars as normal chickens will aim to roost at the highest pint in the coop
2. Roosting Bar Height- are the bars quite high for your type of chickens? I have Bantams and 'Pepsi' one of the chickens is quite small and struggles to get on the bars. So she regularly sleeps in one of the nesting boxes
3. Mites - sometimes chickens might opt for a nest box instead of the roost is that your coop has mites. Mite will hide out of sight during the day in cracks and crevices, such as at the end of roosting bars where they meet the wall. Check for mites by lifting out your roost bars during the daytime and looking at the ends. Its probably not the reason but its always worth checking especially in the colder months.
4. weather- If you live in a colder climate chickens sometimes chickens snuggle together in the nesting boxes to keep warm.
5. Injury- If all your hens are doing this then its doubtful its an injury but its worth mentioning keep a close eye on cuts, infection, bumblefoot, splinters, frostbite etc
Because they are young it wont be age as some older hens struggle to get up on the bars
Hope these help, I'm no expert but these are things ive experienced there are probably many more reasons
Good Luck
