I'm not sure about Warrens or Brahmas, but there are definitely breeds that will be late bloomers...heavier breeds I have heard will lay later than light-weight egg specific breeds like Leghorns and RIRs. My Leghorn was exactly 16 weeks when she layed her first egg, and she is a machine now. She layed one just a few days later that was double yolk! My Ameracauna started a week later at 17 1/2 weeks, which is odd because I have heard that they are supposed to be late bloomers as well but she has always had the most boisterous personality so I'm not suprised. Try going out to pick them up (hopefully they are friendly), if they squat for you (hunker down with their wings slightly out), they are really close to laying. Make sure they know the nest boxes, try putting them in the boxes from time to time to show them where, and make sure you are putting dummy eggs in there to encourage them to lay there, and NOT in the yard somewhere. Make sure they are on layer feed and maybe even extra oyster shell so their eggs come out smoothly and have good hard shells. Make sure that you are giving them plenty of clean fresh water. Laying hens (at least mine) drink a lot more water. Lastly, make sure they are not too stressed by excess noise in the nest area (if you can help it) make sure the coop and run are clean and that they have nice shady places to relax. Heat can take a major toll on chickens so if it is hot where you are they not only may lay later than normal, but they may not be as regular.
