The average age of inland tank barges is calculated by taking an average of the age of every inland tank barge which the company is currently using.
This number is useful for several reasons:
Old ships need to be refitted, scrapped, or replaced, all things which create large expenses and/or large drops in revenue. Having an old fleet when there is a backlog in shipyards is particularly distressing, as it means a company will have to either wait for its ships to be replaced, or pay a lot.
Newer ships are often faster, more fuel efficient, safer, and more versatile in terms of the cargo they can economically handle.