Do you know the way some chicks are hot in their 20s...then in their 40s they're MILFtastic ? But their friend who was also hot in their 20s but looks like Jabba in her 30s ? Whilst your wife was a cutie, but now looks like a crusty aul' hag ?
People are different. People age differently. With regards to sports, injuries, desire, duration of career, nature of their bouts during their career, lifestyle outside of it. It all comes together.
Some people get multiple knee injuries and are fucked. Others reach the top and stay there but their reflexes, speed, stamina all slowly leave them. Some lose the desire they no longer give it 100%, sometimes due to factors outside of the sport, their personal life. It's a mental and physical sport.
In other sports, players dip in and out of form due to divorces, injuries and so on so forth. Some players age like wine, whilst others fall off a cliff over night.