Here is an illuminating study of men deposed in the court of chivalry who were also recorded as serving in the campaigns of 1387-1388 in The Soldier in later Medieval England database.
Since they reported their age and how long they had been armed when deposed, it is possible to get a sense of both the ages of men who were still active soldiers, and when they started their service in arms. This can be compared to John Hardyng's advice that lords' sons should go to war at sixteen.
Although several of the deponents were armed at even younger ages, the median age of first service reported was twenty. It's important to remember that campaigns happened infrequently: a young man might have to wait several years after has sixteenth birthday for even a theoretical opportunity to go to war, and when it occurred he might be serving in a household that didn't go on that campaign.
Here is a record of the Scrope-Grosvenor depositions, with additional biographical information for the witnesses. An interesting picture of the careers of men-at-arms!