I believe you might need to edit the title of your post, from "Glock" to "Flock." I first imagined you might be asking what kind of firearm you might need to dispatch either chickens or predators.
Typical "sad days" for me are: Age 16-17 weeks when I slaughter and process all of the cockerels I don't plan to keep; Any hen who's aging out (for me, 2-3 years) and has stopped paying their room-and-board in eggs; and any sick or injured hen whose treatment and recovery might be futile or more involved than I care to tackle. So except for the cockerels, it's not based on age - it's about my greed and hungry belly.
Most chickens over 18-mos of age will go into molt in the fall and slow down or stop laying until spring. Also, a new chick will start laying somewhere between 20-28 weeks of age. Sooo... if you add new chicks in about February or March, you'll have new layers starting to produce eggs at about the same time the older girls take a break. Your decision when it would be best (or necessary) to remove some of the older hens.