As a side thought, I seem to remember that CT or MT defined multiple
receipts of a Free Trader as a mustering-out benefit for Merchants as each
increasing the age of the ship by ten years (and reducing the amount that
needed to be paid to own the ship free-and-clear by the appropriate
amount): first receipt, new ship, 40 to go; second receipts, ship is now
ten years old with 30 to pay off; third receipt, ship is 20 and pay 20;
etc.
I am pretty sure that is the route by which we made our assumptions.
I will note in one game we had a scout ship that some of the mid-ship cabinet doors stuck at random times. It turned out said 100+ year old ship had a cracked spine and the entire hull was flexing.