Adults can only bear children between the ages of 18 and 50 for vv2,, vv1 its unlimited after 18. I would suggest checking your population because if your at the max then they will not have anymore untill someone dies and makes room, max population for a game without any of the collections finished is 95 and it goes up 5 for every collection you have finished so with 4 collections the max with all finished is 115.

and no your tribes will not try to kill itself, you can have a tribe running for the rest of your life, expecially with the ability to save game files to transfer to new computers.