When did chickens start laying eggs ?

I always thought that chickens were young birds, pullets were almost adult birds and hens were adults and only hens laid eggs.
But now it seems that only chickens lay eggs and hens are never referred to and no one has ever heard of pullets.
I seem to be on a one woman campaign to stop chickens laying eggs ( as they are much too young and this could be construed as cruelty ) and am left wondering why chickens never grow up to be hens these days. Is this an evolutionary phenomenon ?