Van Coehoorn was born in March 1641, one of six sons of Goosewijn van Coehoorn and Aaltje van Hinckena on the Lettinga State in the village of Britsum near Leeuwarden in the Dutch province of Friesland. His father was an officer of German descent in a Friesland regiment of the Dutch States Army and a member of the petty nobility. Menno was educated at home with his brothers, then later at the University of Franeker by his uncle Bernardus Fullenius, where he showed a talent for mathematics and military drawing. In 1657, he was commissioned as a Lieutenant in his father's company, which formed part of the permanent garrison in Maastricht. In 1678, Van Coehoorn married Magdalena van Scheltinga; they had four surviving children, Goosewijn , Gertruda , Hendrik  and Amalia . Magdalena died in 1683 and he later married Truytje van Wigara . He died on 17 March 1704.. How many years old was Van Coehoorn when he was commissioned as a Lieutenant in his father's company?