George HW Bush and Jimmy Carter would each have to live 10 more years to top Ford and Reagan, and Bill Clinton would have to live another 32 years.
Currently there are three living former presidents. From the inauguration of George W. Bush to the death of Ronald Reagan, there were five, which was only the third time that many former presidents have been alive. It was also the longest period with five ex-presidents kicking around -- more than three years, compared with the roughly 15 months between the inauguration of Bill Clinton and the death of Richard Nixon, and the 10 months between the inauguration of Abraham Lincoln and the death of John Tyler.
Three former presidents is a fairly common number, beginning all the way back in 1817, when James Monroe took office. All the previous presidents except Washington -- John Adams, Jefferson and Madison -- were still alive at the time, and in fact all of them survived Monroe's eight years in office.