Eggs didn't hatch, how old of baby chicks can I fool my broody with?

[daniel-delarosa]

Its been 30 days and no chicks. Im thinking of buying some baby chicks from somewhere and swap the eggs out with baby chicks at night. Up to what age chicks can I fool her with?

 

[datchiikasam]

when this happened to me my silkie tried to adopt 3 month old chicks, but i went out and got her 2 babys about 4-5 weeks and she done just fine with them.