Let me give you a link that might help a bit. It does not directly address you question, but I think it gives decent information about the broody process. There is still a lot more to it.
Isolate a Broody? Thread
https://www.backyardchickens.com/forum/viewtopic.php?id=213218
Should they mature before you let them hatch eggs? Two different issues. One, when a hen goes broody, she is acting on instincts. It is really amasing how those instincts kick in. A very young broody may have more difficulties than an older broody, but if one goes broody, I'd not worry about it. An older broody can have bad instincts too. If one goes broody, I give her eggs, no matter her age. They can all mess up or they can all do well.
The other question is, how large are your eggs. When hens first start to lay, the eggs are a lot smaller than they will be in a few months. Those smaller pullet eggs will hatch. I've had decent hatch rates with them. But since the eggs are smaller, they do not contain enough nutrients for the chick to grow as large and strong as a chick would in a regular sized egg. As a result, they are smaller at hatch and not necessarily as robust. If I can get then through the first 48 hours or so, they are as active and healthy as chicks from larger eggs, but I have had noticeably higher mortality rates with those chicks from the small pullet eggs during those first couple of days. Those were eggs from pullets abut 22 to 23 weeks old. Others have said they do not have the same results. I have not hatched enough pullet eggs to say it is definitely a problem, just that when I did it, I had that problem. I'm not going to say I won't do it again, just that I'd prefer the eggs to be larger. If I get a broody and all I have is pullet eggs, she will get pullet eggs.
Another issue. I don't know that the small chicks that hatch from the pullet eggs ever catch up in size to their larger siblings. They have the same genetics, but I just don't know if they catch up. It's not a huge issue with me since I eat most of them anyway and, since meat is a prime goal of mine, I eat the smaller ones first anyway and let the larger ones breed.