5 months old? That's so sad. So sorry!I lost 2 of my 5 month olds this month, so I'll be compiling a post on that. And I have 1 chicken inside that has a limp, but her issues seem opposite of my other 2. I'm hoping she just hurt her leg.
For the one with the limp, I'd soak that one's legs in Epson salt and take a peek to see if there's anything on the bottom of the foot like a black scab. The soak might make it feel better too if it's just a sprain or muscle injury.