What happens when a samurai is too old for battle?

There were a lot of old swordfighters from both Japan and China, but they kept fighting with the young ones, until they died.

Ieyasu Tokugawa (73), Sun Quan (70), Hanzo Hattori (55).

Ieyasu was the unifier of Japan, and still protected his lands at his old age.

Sun Quan was a descendant of Sun Tzu, he controlled part of China, but when he grow older, he gave his power to his son.

Hanzo Hattori was a ninja working under Ieyasu, he fought up until his death.