I've updated the shipping lines post with the following:
"When a new ship is built, the shipping line will retire the oldest ship of the same general type (with a minimum of 10 years older) that is not currently carrying cargo. So its possible a civilian ship will last longer than 10 years, due to always being active when the shipping line is looking for a ship to scrap. As a ship will not have cargo more than 50% of the time, its luck will probably run out at some point.
All civilian ships will be scrapped when they reach 15 years old, regardless of whether a new ship has been built.
With these changes, a shipping line is generally going to have a relatively modern fleet, with the oldest ships ranging from 10-15 years."
Steve