I may be changing my opinion on this. I'm planning to put some pullet eggs under a broody tomorrow to see what happens. My pullets are exactly 5 months old.
Pullet eggs are viable. They can hatch. I've never doubted that. I still think the hatch rate will not be quite as good as with full sized eggs, but some and probably most should hatch. There are not enough nutrients in those small pullet eggs for the embryo to grow to its full size before it hatches. Any chicks that hatch will be smaller than those from a regular sized egg. Since they have been nutrient deprived while growing in the egg, I'd expect them to be weaker and more susceptible to disease or just the elements, something like undernourished babies. That's what they are. That's mainly why I don't think as many will hatch, some will possibly be too weak to make it through the rigors of hatching. There is a possibility some will be deformed also, again because of that lack of nutrients in the egg. The extension sites recommend hatching normal sized eggs, not the small ones or the unusually large eggs.
The question I am not sure of is if they will make up for their small start or will they always be runts? I don't know the answer to that. I'm aware that hatching out one batch is not a statistically sufficient number to give a definitive answer, but it is the best I can do. What got me to thinking about this is that I learned commercial operations that breed the chickens that become the parents of the commercial meaties put the hens with the roosters at 22 weeks and start incubating eggs a few days later. Since they severely cull, they will not accept runts.
If you are willing to accept the chicks will be smaller and possibly have a worse hatch/survival rate than if you wait, you can hatch the pullet eggs. You should get several chicks out of them. If you are raising them for meat, you might be at a disadvantage, but if you are out for eggs or pets, I'd think you will be OK with the ones that make it.