The Hero Who's Never Been Defeated - Beowulf vs Grendel
The Hero Who Discovers that He Can Be Defeated - Beowulf vs Grendel's Mother
The Hero Who Knows that He Can Be Defeated - Beowulf vs the Dragon
Seen in this light, the story segments of Beowulf become a natural progression, one naturally leading to the other. While these mirror age (as discovery must mirror age), his age really isn't the focus. The experience is the focus.
The story seems to ask: which is the best hero? Which is real courage? I think that the story answers its own question. The hero who knows that he can be defeated is the greatest hero because he conquers his own fear and fights anyway. When Beowulf fights Grendel, he has no fear. When he fights Grendel's mother, he learns fear. When he fights the dragon, he fights despite his fear, praising the nobody who was the only one of his retainers bold enough to fight the dragon with him.
In that respect, we could say:
The hero who has never known fear.
The hero who discovers fear.
The hero who has conquered fear.