The old farts have it

Age and treachery, it is said, beat youth and X every time. But what is X?

Googling suggests that this part of the proverb isn't very stable. I find: enthusiasm, skill, idealism, enthusiasm, innocence, inexperience and experience, endurance, exuberance, ability, virility, skills, reflexes, speed, talent, baggy pants, and a bad haircut.