1030 - March 17, 1058: Age 28
Lulach was unfortunate in his mother: she was Gruoch of Scotland, also known as Mrs. Macbeth. Gruoch was more than just a stereotype of feminine ambition: she was also the granddaughter of Kenneth III of Scotland. Macbeth, Lulach's stepfather, was also the grandchild of a king, in his case his mother was a daughter of Malcolm II.
It's good to be the king, unless of course you are a king of Scotland, in which case your life expectancy is historically rather short. Macbeth, surprisingly, ruled a full 17 years before being killed by Malcolm, the eldest son of that same Duncan who Macbeth murdered to get the throne. Malcolm had been sent out of Scotland for safety on the death of his father in 1040; he was only 9 at the time.
In 1057 he returned, a grown man, to take back the throne. Even when he killed Macbeth he was not yet successful, as Macbeth's followers hurriedly crowned his stepson Lulach. Lulach (also known as "Lulach the Fool") ruled only a few months being killed "by treachery". Macbeth was out of stepchildren, and Malcolm ascended the throne as Malcolm III.