I was told the longer the hens wait, the better. Our first one was 19-20 weeks, I think, but we still have hens that haven't laid, and they just turned 26 weeks on Tuesday. Out of 18 hens, 6 are not laying.
If they start laying early, you have a greater chance of issues like egg binding and prolapsed vent, which is what I am faced with this morning - awful awful, I do hope our hen makes it. She was one of the earliest layers.