No worries, that's on the low side of normal. It will depend to some extent on the breed(s) that you have, also the shorter days of fall and winter can slow things down. You can help by making sure they have a high protien layer ration and plenty of fresh water. If you scrimp on feed it will effect laying ability. Have any of your girls gotten redder in the waddle and comb yet? They usually darken up a week or two before they start to lay. You might also notice them practicing in the nest box and/or squatting when you walk by or try to pick them up.
You didn't say what breeds you have-- I have Barred Rocks that started at 24 weeks, a RIR @ 25 weeks and my BO girl is in the nest box , today, for the first time--she is 28 weeks old. My EE (24 weeks old) and Ameraucanas (28weeks old) haven't laid a single egg...
