Hiroshi Hoketsu (who turns 71 on March 28) earned the spot by winning an international dressage meet in France earlier this week. It remains unclear whether he will actually compete in the games, but Japanese Equestrian officials not only exist but will also announce Hoketsu's participation status in the near future.
Hoketsu was already Japan's oldest olympian in the Beijing Games, when he competed as a spry 67 year-old. He's got some work to do if he wants to be the oldest Olympian ever, though.
The oldest Olympian in history is Swedish shooter Oscar Swahn. He won a silver medal at the 1920 Antwerp Olympics at 72, his sixth medal in three games.