Edward III (early 14th century) 'fought' at the Battle of Halidon Hill when he was only 21. I put quotes around 'fought' because he was in the center of his battle line but the Scottish broke and fled before they could get into hand-to-hand combat with the English.

His actions in later battles and wars would lead one to believe that he would have been in the thick of things had the Scottish not succumbed to the English arrows.

If you are referring to a lesser noble leaving home on a crusade or something, I imagine the chances of taking part in face to face combat only go up. Robert II, Count of Flanders, was about 30 and Tancred (later Prince of Galilee) was around 20 when they joined the 1st Crusade and took part in a number of major battles.