Old English colt "a young horse," also "young ass," in Biblical translations also used for "young camel," perhaps from Proto-Germanic *kultaz (source also of Swedish dialectal kult "young boar, piglet; boy," Danish kuld "offspring, brood") and akin to child. Commonly and distinctively applied to the male, the young female being a filly. Applied to young or inexperienced persons from early 13c.
COLT'S TOOTH An old fellow who marries, or keeps a young girl, is ſaid to have a colt's tooth in his head. ["Classical Dictionary of the Vulgar Tongue," 1796]
The image is in Chaucer. Colts shed their first set of teeth beginning at about three years.