Mark Twain at age 31.

Mark Twain (Samuel Langhorne Clemens)
(November 30, 1835 – April 21, 1910)

Clemens was apprenticed to a printer at age 13 and later
worked for his older brother, who established the Hannibal
Journal. In 1857, the Keokuk Daily Post commissioned him
to write a series of comic travel letters, but after writing five
he decided to become a steamboat captain instead. He signed
on as a pilot’s apprentice in 1857 and received his pilot’s
license in 1859, when he was 23.

Clemens piloted boats for two years, until the Civil War halted
steamboat traffic. During his time as a pilot, he picked up the
term “Mark Twain,” a boatman’s call noting that the river was
only two fathoms deep, the minimum depth for safe navigation.

When Clemens returned to writing in 1861, working for the
Virginia City Territorial Enterprise, he wrote a humorous travel
letter signed by “Mark Twain” and continued to use the
pseudonym for nearly 50 years.