Fully grown?

[Whirlwind]

My 3 BR pullets were switched to layer feed at around 16 weeks and they are laying fine. That may of been a couple weeks early but that was when I bought them and the man had just switched them.

 

[SpottedCrow]

Slifer, my Light Brahma didn't reach her full size until she was OVER a year old, almost 2.
Your light breeds will mature faster and reach their full sizes by a year in most cases.