What indicates that Beowulf, though old, retains much of his strength/

Beowulf is basically on top of the dragon fighting to the end. He stabs the dragon so hard that his sword snaps in two. The dragon lands a bite on Beowulf’s neck and blood gushes out. Still the near dead Beowulf fights on. The bite is venomous but Beowulf fights as long as he can finally taking down the beast. Not bad for an old man!