We bought baby chicks this spring. They haven't begun laying yet, and they are about 20 weeks old now. We have 3 buckeye and 3 brown leghorn hens. It has been a very humid summer in Wisconsin this year. Why aren't my hens laying yet?
Chickens usually start laying around 5-7 months old, so yours are actually at the early end of that spectrum for now, so I wouldn't worry yet. Buckeyes, being dual-purpose birds, will probably take at least a couple more weeks to begin. Brown Leghorns should start laying before them, probably in a couple weeks as well. When they get close to laying, they'll begin "squatting" for you when you reach down to pick them up/pet them, and their combs and wattles will get larger and redder.
The simple answer as to why they are not laying yet is that they are not ready yet. At 20 weeks of age you are just on the early end of the time frame in which you might expect to start seeing eggs. It may actually be weeks yet and you would still be in the normal time range for them to start laying.