It all stems from the year F1 started - 1950. The world had just come out of an horrific 6 year war which had devastated Europe (the main continent for this type of motor racing) and killed millions of young men. So really, there weren't as many men (because why let women race) of a young age ready to jump into the top level of racing. Most of the drivers were all pre-war racing veterans who at least knew how to drive a car, and there hadnt really been enough time for junior series's to start up and bring along a new generation. It took a long while for drivers in their 20s making a debut to become regular, and older drivers rarely get looked at nowadays. It's become "if you're fast enough, your old enough", and once drivers hit like 25 and haven't made the jump to F1, they begin to look elsewhere because it's like "you've had your chance to find a seat and you didn't. Off to GTs or Indycar for you". Drivers need to prove themselves in their teens or someone younger and faster will get that seat before you.