Before I go on wanted to apologize, if anyone felt affronted by the "age-limit world". I actually didn't think about the moral implications at that time. How to topple that issue?
Whether a group wants to get into the moral realms of "mass murder" or not is up to their own bidding. I always thought of sci-fi and fantasy as a possibility to discuss themes that couldn't be discussed in "real life". Maybe it's me but mass murder never bothered that much in these kind of stories. We're talking about a world, where 3.000 years ago a huge part of the world was just shattered apart.
To the question of feasibility: This is highly dependent on the circumstances people these live under. If they have a functioning society, enough food and water, a mild climate and a moderate amount of diseases it would be. With a 0% death rate a women would just have to make 2-3 children in about 11 years.
The moral questions of young mothers and young death is the far more pressing one and I can't answer that for you.
What the group does or doesn't do... well I always figured they should be left to their own devices but any righteous group would at least try...
Anyway, since the main goal of the campaign is to reweave the realms they would put an end to the practice anyway.
Well... About linking the age limit realms, if the young ones shouldn't know where kids come from, they have to be much younger. Put some fourteen year old kids together and they'll figure out how kids are born 9 Month later.
More world ideas:
A realm where most adult women where out of the realm as the sundering happened (because of some wedding ritual) the remaining adults thought that a demon had stolen almost all women and trapped them to take the rest. So they disguised all girls and the remaining women as boys and men. Today females and males are virtually indistinguishable, if you don't know what you have to look for. Tradition calls for clothing that would make it hard to distinguish by typical gender characteristic, but there are secret clues.
A kingdom where mighty sorcerers conjured demons to do there bidding, but after the sundering no demons could be called anymore. The demons couldn't find their way to the realm. To keep in power the leaders imposed slavery, but slavery of humans was disallowed, so the human slaves had to dress up as demons. Till this day all slaves are painted in colors and have fangs and horns put on their bodies.
A divided realm, with only a small path between them, either this was one realm that was almost ripped apart or two totally different realms that now lie near each other in oblivion. One of the realms has almost nothing. Before the sundering it lay on the edge of the fruitful plains toward the northern regions. Only rich trade made live bearable. They think a mighty demon, the "sunderer" destroyed the world.
The other realm is much more livable. With a wonderful river and warm climate people have a paradise in this realm. They also think that a being called the sunderer is responsible and has to be kept in good mood.
So: Every 20 years the first realm sends a fighter to hold the sunderer at bay, so he can't destroy the last piece of the world. This fighter goes into the second realm, where he assumes the role of the sunderer and has an awesome time. 20 years later his replacement arrives and he goes back to the first realm and tells them how he had to fight the evil sunderer. The replacement guy tells the second realm that he is the rejuvenated sunderer and they do his bidding.
A realm that is controlled by mighty sorcerers. Only these sorcerers have no clue about magic (at least not the fantasy kind). Before the sundering real wizards controlled the land but over time almost all knowledge was lost and today only parlor tricks remain. But everyone fears the mighty magician who can go through walls, can be sawn in half or even let a tower vanish! FEAR THE MIGHTY WIZARDS!
If you like the whole "drifting" idea. How about some kind of "assorted house"? A realm that was clotted together from small shatters, that didn't fit anywhere else. It's a huge area, some square miles or larger made out of the interior of houses. Sometimes you there might even be some kind of winter garden or courtyard but it is mostly rooms.
I have three other ideas about a battlefield, hunters and a never ending party but they will have to wait until later.