There are a lot of things that impact fertility in CK2.
Yes, women are on a timer, so to speak. But I've gotten some kids cranked out at age 40. Not common, but it can happen.
The catch is that men also have a variety of things that can impact their fertility, too. Illness, stress, depression, celibacy, chastity, infirmity, and even being incapacitated (by either injury or old age, requiring a regency). That 70 year old king of yours probably ain't going to get too much babby-forming done unless he's really healthy. Old age hurts more in CK2 than in CK1 and DV.
Oh, and you'll notice I mentioned celibacy. Obviously, if you are controlling yourself, you won't choose celibacy if you need more heirs. But you can't control your sons, and I've had heirs who decide it is better to be celibate than to burn.
EDIT: There are also special limitations on unimportant characters. If you check out the game files, there are child limits imposed on non-heir barons, and non-dynasty, unlanded courtiers. That keeps the opposite problem, a court full of stupid characters, from happening.