My chickens just turned 18 wks. old but are not laying eggs yet. They are Buff Orpingtons and Brahmas. On the Purina bag it says to switch them to layer at 18 wks. Should I switch them to layer now(at 18 wks. old) or should I wait till they start laying eggs? I don't really want to wait till they start laying because then I'll have to buy another 50 lbs. bag of Purina Starter, and then what do I do with all the extra if they start laying eggs in the middle of it? I am just about running low on the Purina Starter now.