It's probably the older age. The older a hen gets, the more likely she'll go broody. But because of her breed, I'd keep a VERY close eye on her around the chicks for the first couple of days. You never know when you got a bad broody. After she's been sitting for at least a week or more, try sneaking one chick under her. If she accepts it, place the others under her within 6-12 hours of her having the first. Keeping a very close eye for the first couple of days.She's a production blue,which they shouldn't even be going broody but so far 4 out of the 5 have gone broody multiple times and they're only 2 years old.