Well, with 23 hens in 5 different breeds that all mature at 20-23 weeks here is what I have learned. Note that this is just what is going on here not what will happen everywhere with every hen.
Mine started getting the red comb and wattles, some got larger comb and wattles, beginning at 17 weeks.
The first hen to lay and egg was just over 17 weeks, probably way too early, she successfully laid one small egg then, the same day, tried to lay again. There were two eggs together (individual but at the same time) that had no shells and resulted in a very severe prolapsed vent. A friend of mine who is a vet assistant came out and basically said there was no chance to save her and we had to put her down. She is now the first in our little poultry crematory. There is nothing you can do to prevent a hen trying to lay early but you can hope for the best.
Just prior to 20 weeks old one of my Columbian Wyandottes laid her first egg. Very small, not even a gradable size. But a perfectly forms, pale pinkish/beige egg that tasted great. From this point she has laid an egg each day and is now up to what would be graded Small. She is supposed to be a large brown egg layer so I am guessing things will continue to change as to size and color.
Three days past the 20th week a second hen, a Silver Laced Wyandotte, began to lay. Again a very small pinkish/beige egg to start but one each day and, if as with the first hen, they will increase in size as time passes.
My other 21 hens, Silver Laced and Columbian Wyandottes, Barred Rocks, Partridge Rocks, and Blue Andalusians, have not laid their first egg yet and they will all be 21 weeks old this Friday. They all, except the Andalusians, have developed the red comb and wattles. The Andalusians comb and wattles are getting larger but are still, except for one, a pale pinkish grey color. So they may extend to the full 23 weeks or more.
So, lessons learned, don't rush your hens, don't add extended light time this time of year, do feed them well and let them free range frequently. Patience is a virtue and is essential in raising hens for eggs.