If it were me, I'd wait until they were 20 weeks old or started laying, whichever comes first. It won't hurt them to stay on the grower feed until then.
You might want to put out oyster shell for them as they near possible laying age. I noticed that my girls started to eat oyster shell 1-2 weeks before they laid their first egg. It was a very reliable predictor of future egg production. That way, they never have any kind of calcium deficiency, less chance of soft/no shell eggs.