Those recommendations on when to start Layer come from commercial operations that control when they start laying, mostly by manipulating the lights. They want to delay egg laying a bit to give the flock more time to mature so the eggs are a bit larger when they start laying, those eggs sell for more. Also delaying laying helps stop a lot of those weird pullet eggs we often see when they start laying. Those weird pullet eggs don't work well with their machinery. So while ISA Browns can start laying earlier they don't start Layer until 18 weeks and egg production starts later.
You are not raising them that way by manipulating lights so you are not sure when they will actually start laying. It could easily be anytime from 16 weeks on but could be delayed some. I'd suggest you either wait until you see the first egg or about 16 weeks offer oyster shell on the side and keep feeding them whatever you are feeding them, at least until that bag of feed runs out. Many of us never switch to Layer but feed a low calcium feed and offer oyster shell on the side.
