The notion of retirement has been around since the 18th century when life expectancy began to increase beyond an average of 26-40 years of age, and since then various factors have led to where we are with it present day, but basically the idea was that at some point in their careers, age and health factors were a drag on an employee’s productivity and it served employers to get them out of the workplace in favour of younger replacements. Even if they had to pay them to do so.
The world of professional sports is no different. Though a fair bit earlier than in many other fields, at some point athletes find themselves in that position where they just aren’t as effective as their more youthful or more healthy counterparts.