A couple of mine started early at 14. Many start at 16-20. Others start at 20-28. A lot of it has to do with breeds.
I would disagree with countrygirl on the feed. If they still aren't laying they could use the extra protein (20%) found in a grower or all flock. That said, be sure to offer additional calcium (in the form of crushed oyster shell, available at the feed store) as your hens may need it if they're getting close.
The extra calcium in a layer feed (~3%) is great for laying hens but can be harmful to hens who aren't yet laying.
Watch for the "squat". When they're doing it, you've got about a week to wait.