Morris III (Prince of Caledon)
Morris the Third, fourth son of Donald IV, was fourth in line after the death or retirement of his three older brothers (Ian IX, Michael II, Fergus II) - so he didn't accede to the throne until he was nearly 50.
His first wife, Ann Sedgwick (24th Baroness of Slighrood) a famous socialite, was three years older than Morris. The couple had four daughters and a son, Laurence. Historians also believe Ann was mentally ill and suffered from a drug and alcohol habit which - forensic historians believe - led to her death in 654. By this time, Morris had already been carrying on an affair with the 21 year old Colleen of Doncaster; they were wed within a year of Ann's death, and Morris' second son, Ian, was born 27 weeks later (although the media at the time carried the story several months later).
Citing ill health, Morris stepped down at age 59, turning the throne over to Laurence.