There are currently four living former presidents: Jimmy Carter,H.W. Bush, B. Clinton,G.W. Bush. The two older statesman are both 91 born in 1924 about four months apart. The two younger statesman are both 69 born about one month apart.
Nixon and Ford were born in the same year(1913) though 7 months part. Nearly hundred years earlier Grant and Hayes were born in 1822. Lincoln(Feb. 1809) and Johnson(Dec. 1808) were close just a couple of months apart but in different years. Andrew Jackson and J.Q. Adams both born in 1767.