It's not a specific age, it's a seasonal thing, happens every year, in the fall. But if your birds are very young when fall rolls around, they may not moult the first year, because they don't need to yet. In that case, some breeds will moult the following spring, most will wait until the...
I'm betting they're beginning to moult. Wait a while. Increase the protein. Some breeds don't show the moult as obviously as others. In another week or two it will probably look like they've been having pillow fights when you weren't looking.
Mine are moulting, egg production is down, some...