The average age of fleet is calculated by taking an average of the age of every vessel 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.