when did he join the military?
August 16, 1791
William Henry Harrison
On August 16, 1791, Harrison was commissioned as an ensign in the Army in the 1st Infantry Regiment within 24 hours of meeting Lee. He was 18 years old at the time. 
who is william henry harrison? An American military officer and politician.
wiki:19622834
William Henry Harrison [SEP] Early military career
On August 16, 1791, Harrison was commissioned as an ensign in the Army in the 1st Infantry Regiment within 24 hours of meeting Lee. He was 18 years old at the time. He was initially assigned to Fort Washington, Cincinnati in the Northwest Territory where the army was engaged in the ongoing Northwest Indian War. Harrison was promoted to lieutenant after Major General "Mad Anthony" Wayne took command of the western army in 1792 following a disastrous defeat under Arthur St. Clair. In 1793, he became Wayne's aide-de-camp and learned how to command an army on the American frontier; he participated in Wayne's decisive victory at the Battle of Fallen Timbers on August 20, 1794, which ended the Northwest Indian War.