"Following tradition, all former presidents are invited to attend the ceremony on Jan. 20 when Trump will be sworn in. But not all living presidents have consistently attended the inaugurations of their successors.
President Barack Obama in 2009 enjoyed the presence of all four living presidents— both Bushes, Carter and Clinton — at his first inauguration, but the Bushes skipped Obama’s second event in 2013, citing George H.W. Bush’s compromised health.
George W. Bush also did not have a full slate of former presidents at his second inauguration in 2005, as Gerald Ford skipped the event. He was 91, and died the next year.
For Trump’s inauguration, the Clinton camp has been especially divided over whether Bill Clinton should accept his invitation as a former president."