Aside from those dying in office, including assassinations, which U.S. president died at the youngest age?

James K. Polk, the 11th U.S. President from 1845-1849 died 103 days after leaving office at the age of 53 years and 122 days. Polk served for one term, but much happened during this time.

Born in a log cabin, the first of ten children to a well-off farmer in 1795, Polk was frail and suffered from urinary stones. An operation to remove them, without the benefit of anesthesia, might've left him unable to sire children. He had a rather meteoritic rise in politics . He was elected the clerk of the Tennessee State Senate in 1819, and subsequently became a Congressman. He became Governor of Tennessee and then won the Presidency in 1844 as a Jacksonian Democrat.

The United States expanded greatly under Polk. He settled a territorial dispute with Great Britain, annexing Oregon and avoiding war. The annexation of the independent Texas, however, inevitably led to the Mexican War. California and other South Western states were gained but this led to the expansion of slavery. Polk, a slaveholder his entire life, was opposed to the limitation of slavery and pushed for its growth into the newly acquire territories. Putting territorial expansion and economic growth above resolving the slavery issue led to increasing divisiveness that ultimately led to the Civil War.

Honoring a campaign promise, Polk refused to run for office again in 1848. Exhausted from his term, Polk died from cholera contracted during an epidemic and died on June 15, 1849.