A young hen CAN lay eggs as young as 4 months old, but I wouldn't recommend letting a bird that young raise babies. She (and He) need to mature some. I like to let my birds get as close to a year old as possible. I would say AT LEAST, 9 months old at the earliest.
It's a good idea to let young hens sit on dummy eggs a few times to make sure they have the hang of sitting long enough to hatch out any babies. Sometimes you'll find that the young ones get "bored" and leave the eggs before they should. With dummy eggs you don't have to worry about them leaving a baby that's about to hatch.
Some young birds are smarter than others. Some are not. Those birds don't know what to do and can just abandon the eggs. I've observed that. I also have this very smart hen that raised 1 baby at 4 months of age. She was like a pro. Her baby is pretty smart, too. The dad is also smart. I suppose it runs in the family. To be safe it is better that they mature first and not breed them early.