Like most animals, roaches do have a reproductive lifespan. I'm not sure what it is, but I suspect it's around a year old since that's when I had a sudden downturn in my colony's production. I add new adult roaches every six months to stay on top of it, mostly males since I cull younger males pretty quickly.
The cause of the young-adult deaths could be the food. Normally when I see the deaths of young adult roaches, it's because the protein in their food is too high. Most bug chow available is designed with crickets in mind, who require a high amount of protein. But roaches aren't very good at digesting protein. If they eat too much, it becomes uric acid (urates, the same stuff in reptile poop) and starts getting loaded in their kidneys. Young animals use up protein better since they're still growing, but adults succumb to it.
This can also affect our animals. In reptiles, high uric acid causes gout and other kidney problems.
For roaches, their food protein should be around 12%.