Patience is the ticket. You don't mention the breed(s), but that is a factor too. I have seramas which are very early layers (usually by 4 months). I also have large fowl brahmas, very late layers. With my very first flock of brahmas there was a wide span of time between the first to lay, 18 weeks old and the last two to lay, 38 weeks old.
When you see the combs and wattles getting red and the pullets squatting for the roo (or you, if they have no roo) you'll know they are getting close.