When do they become hens

[kside001]

The same physiological reason that you're called an adult at 18 years old. It's just an arbitrary number.

 

[Farmette boy]

I looked this up on Wikipedia....makes sense:

Females over a year old are known as hens and younger females as pullets[8] although in the egg-laying industry, a pullet becomes a hen when she begins to lay eggs at 16 to 20 weeks of age. In Australia and New Zealand (also sometimes in Britain), there is a generic term chook /ˈtʃʊk/ to describe all ages and both sexes.[9]