About how old is Beowulf when he fights his last battle in Beowulf?

In the last section of the story, after Beowulf returns to
his homeland, it is revealed that after Hygelac's death, the "kingdom came into
Beowulf's hand for fifty winters."  With this piece of information it would be safe to
assume that Beowulf is at least 70 to 80 years old when he faces his last foe -- the
fire breathing dragon.  When you consider the average life expectancy of a male 1500
years ago it is actually quite remarkable that he is alive at all -- especially
considering how dangerous his life would have been.  It is additionally remarkable that
Beowulf fearlessly goes into battle to save his people from the danger of the dragon. 
He is a very old man, yet he and only one other man are emotionally strong and brave
enough to face this enemy.  He dies from wounds received, but he faces his death with
valiant dignity.