If you have different types of pullets, you might find that they start laying at different times...18 weeks, 20 weeks, even longer. One good alternative to switching to layer feed too early (it has too much calcium for growing pullets) is to continue to feed the grower feed, but maybe around 16 or 18 weeks attach a small dish of oyster shell to the side of the run. Those that need the extra calcium will help themselves to the oyster shell and those that do not yet need the oyster shell won't want it.