There has been some discussion on that. Pullet eggs are smaller so they do not hold enough nutrients for the chick to grow as big as a chick developing in a regular sized egg. They have the same genetics, though. There is debate as to whether they ever catch up with the chicks from regular eggs in size as they grow.
I tried hatching pullet eggs once. The pullets were about 20 to 21 weeks old, just mutts from dual purpose birds. A hen went broody so why not. The eggs hatched real well, 8 out of 10. But I lost 4 of the chicks before they were a full day old. How much of that was due to maybe something the broody did (she was not an exceptionally good broody raising the chicks), maybe that they were small and not as robust, or maybe it was just one of those hatches. I honestly don't know. But I have never had mortality like that with chicks from regular sized eggs. The ones that made it past the first day were healthy, active, normal chicks, just a little small. I ate all four at about 20 weeks of age and they were a little smaller than I would normally have expected. Would they have ever caught up in size? I don't know. One time hatching pullet eggs certainly is not a statistically relevant experiment.
Something else I've read is that some pullet eggs seem to have extra thick shells. That thick shell can interfere with a chick hatching. I think that was on the Florida incubation troubleshooting article. I certainly did not have that problem however. Just another one of those things that can happen, not something that definitely will.
The extension sites usually say to not set small eggs. Other than that bit on shell thickness, they don't say why. I think it is because the chicks that hatch are small. The smaller eggs tend to hatch a bit earlier than the regular-sized eggs. Maybe that has something to do with it.
I saw a video on managing the parents of the broilers. That specific commercial operation started incubating eggs from 23 week old pullets. These were for broilers, not layers, and the chickens had been specifically fed to produce eggs for hatching broilers.
So when can you incubate? Whenever you want. If you start early rather than late, you may or may not get higher mortality. I truly don't know. The chicks will hatch smaller than normal but will have the same genetics as chicks from larger eggs. It may be a little harder to choose which are your best breeders since they might or might not wind up smaller as adults.
