My white leghorn was my first one to lay. Now, I have read anecdotal stories on here of white leghorns laying at 15-16 weeks, usually around 17-18 weeks for them, though. Mine was 21 weeks when she dropped that first egg, but I got my birds from questionable people (didn't realize it at the time) and NONE of my birds is on schedule as far as age/development goes. Three of my flock girls are 26 weeks & one is 30 weeks, and none of them are laying! The three that are 26 weeks don't even have combs and wattles yet, but by all accounts SHOULD already be laying! My 30-week-old EE has been squatting and singing the egg song for almost TWO SOLID WEEKS, and still nuttin'. So the "when do they start laying?" question is a completely loaded one.
Ask 5 chickenkeepers that same question and get at least 8 different answers. LOL.