When to switch over to layer feed? 10 points! thanks!?

My pullets turned 16 weeks on last friday (June 18th) and it is time to stock up on feed. Should I continue to feed them chick started until they lay their first egg or what? They are large breed/dual breed chickens (Barred rocks, Orpingtons, easter eggers, austrolorps, marans, delewares, jersey giants). Thank You very much!
 
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