What breed are they? If they are a breed that often begins laying at 18-20 weeks of age (like Leghorns, Red Stars, Black Stars, etc), I don't think that it would hurt to start them on layer feed. It will help prepare them for laying later on. However, I'd hesitate before starting to give layer feed to birds that likely won't start laying for another month or so (many dual-purpose breeds like Wyandottes and Rhode Island Reds won't start laying until they are a little older).
Hi Huntered. If you are getting pullets from Moyer's they will probably be laying in a week or so after you get them. I know mine only took a week from when I got them. Layer would be fine at that age.