Try reading the manga over again Pillars are rare and special. There were only nine of them when the manga started. After Kyojuro died months have passed and he still doesn't have a replacement. In chapter 134 the cannon fodder starts moaning about how much better the pillars are and how hard it is to become one and why there are so few.
You make the requirements sound like something easy to accomplish but its not. If you fight one demon every month it would take a little over four years to kill the necessary number of demons. And surviving those battles is no joke, chances are usually against you and people often die on their missions not to mention the risk of being losing limbs, or being maimed. As for defeating one of the 12 Demon Moons. You saw how much people died when fighting the Mother Spider Demon on Mount Natagumo and she wasn't even close to being a lower moon. There is a huge difference between the bottom and the top.
Uzui retired at the age of 23 because he lost an arm. Not a great example.
A persons physical peak lasts between 20-30 years of age usually. So no, athletes usually do not start losing their edge at like 25.