This was a discussion I had with a client just a few days ago. He wasn't overweight, he walked daily and did some minimal bodyweight exercise occasionally. He asked if I knew why aches and pains in joints, etc. start to really pick up around age 40 or so.
So just going back to my learning of physiology, the human body really hasn't changed for thousands of years. If we're looking at it objectively, like any other animal we instinctively look to procreate to carry our genes down line. In the early human years on Earth, all we did was what other animals did. Females start to have periods about 10-13 years old. Obviously at that time, they can get pregnant. Males are also high in testosterone around the same age. IF the goal was to just rear children and feed them, then this would be a good age because there's less complications with birth and one is still young enough and fleet enough to gather/kill for food and feed the family. Ev en today, most people's peak physical performance was in their teens and into their mid 20's or so. Without medical intervention, what is the average human life expectancy barring being killed by accident or murder back then? Probably right around 40-45 years old (a guess). So by the time you hit 40, the body is getting worn out. You don't run as fast and you likely don't see as well either. If you're the male leader of the group and can't lead as well, normally a younger male will challenge and likely beat you out. And so on and so on.
Of course now with medical intervention and technology, along with understanding how cells work, better option for food for complete nutrition (not just berries and freshly killed meat), and physical exercise, we've obviously been able to extend human life expectancy to much much higher years. But also look at how much less physical one becomes and how much more fragile we are in those later years.
So why 40? IMO (and based on evidence) it's because the human body likely wasn't made to last too much longer than that naturally.
A.C.E. Certified Personal and Group Fitness Trainer
IDEA Fitness member
Kickboxing Certified Instructor
Been in fitness for 30 years and have studied kinesiology and nutrition