This idea of the young man summing his father's "count" takes us back to the metaphor of beauty as "treasure" in line 6. Again the big idea here is that beauty becomes a lot like money, and turns into the kind of thing you can save up and pass along to your kids.
- Line 11: Instead of wasting his beauty by keeping it all to himself, the old man has deposited it with his son. The son can prove that his father's beauty isn't really gone, it has just been handed down to the next generation. In this way, the son evens out his father's accounts, and covers his debts to the world or (makes his "old excuse"). This language is a bit unusual, but keep in mind the main idea of this line (beauty = money), and you'll get the point.