Last year one of my sebright hens went broody. She was around 5 months old and had only been laying for a month. Out of 8 eggs, 4 hatched. 6 were fertile, but one cracked open, and another didnt hatch.
I have hatched 17 eggs from my incubator, and have had 13 hatch from my hens, that were ALL pullet eggs. The thing about pullet eggs is that sometimes they can be small, or not shaped properly, and this would make it hard for the chick to hatch. I don't see any problem with letting her set.