Who was the oldest athlete to participate in the 2008 summer Beijing Olympics?

Equestrian Hiroshi Hoketsu of Japan.

Hiroshi Hoketsu was 67 years old when he competed in individual and team mixed dressage at the 2008 Games in Beijing. The 2008 Games was his second Olympics ... he also participated at the 1964 Games in Tokyo! The 44 years in between Olympic appearances is the longest in Olympic history.

He had qualified to compete in the 1988 Games in Seoul but was unable to when his horse was quarantined.