Okay, I see your problem now. First off, if you put a male on breeding, all the females will get pregnant. So, it is better to have the females as the breeders.

More importantly, have you upgraded your medicine tech levels. If you are on level 1, the adults are termed elderly in their 50's and start dying. Upgrading medicine by one level makes them die in their late 60's and early 70's.

Hope this helps.