We often don't run 1 year old ships and aircraft in regular traffic as they break down too often. Other nations do run commercial aircraft that are 30-40 years old.
Sure:
BIMCO is reporting that for the first time in over a decade, boxships have reached a new peak with an average age of 14.2 years. Over the past 13 years, they report the average age has increased 4.3 years from when the average age hit a low of 9.9 years in August 2010. The current average of the ocean-going vessels is the highest of the three main shipping sectors, with the dry bulk fleet average age standing at 11.9 years whereas tankers on average are 12.8 years old.
https://maritime-executive.com/arti...t-average-age-despite-newbuilds-bimco-reports
Major commercial ships are taken out of regular service after 20 years or so.
Boasting one of the oldest passenger aircraft is Canadian Air Inuit with four 737-200's averaging an age of 44 years. The airline also has a 737-300 aged 32 years and 15 Dash-8's at an average age of 29 years. Three 737-800s aged just 11 years held bring the average fleet age for the airline down to 29.4 years, Jet2 still flies six 757-200s with an average age of almost 33 years!
So, that is exceptionally old at 33-44 years?
What's the average age of jet liners in regular traffic?
From Oldest to Youngest: The Average Fleet Age of the 10 Major U.S. Airlines
Allegiant Air -- average fleet age: 19.8 years
...
Spirit Airlines -- average fleet age: 6.6 years
https://www.fool.com/investing/2017/07/03/average-fleet-age-of-the-10-major-us-airlines.aspx
I am not sure of how you are working your maths. A 25 year old ship has 15 years left to run on the mortgage. Assuming a linear depreciation it would be worth 15/40 of its original purchase cost which means a shade over 19MCr to buy off the mortgage outright. I am not sure what you are using for the idea of an "original down payment" as the rules have no "Down Payment" mechanic.
The original mortgage was:
LBB2'77, p5:
STARSHIP PURCHASE
Bank financing is available to qualified individuals for the purchase of commercial starships. After a down payment of 20% of the cash price of the starship is made, the shipyard will begin construction of a specific vessel. Upon completion, the vessel is delivered to the buyer, with the bank paying off the purchase price to the shipyard. Because the bank now holds title to the ship, the price must be paid off in a series of monthly payments to it. Standard terms involve the payment of 1/240th of the cash price each month for 480 months. In effect, interest and bank financing cost a simple 120% of the final cost of the ship, and the total financed price equals 220% of the cash purchase price. The loan is paid off over a period of 40 years.
In addition, the bank will insist that the purchaser submit an economic plan detailing the projected activity which will guarantee that monthly payments are made. Unless a character has some form of guaranteed income (perhaps large royalties from some property he owns), these conditions will generally rule out purchases (at least financed purchases) of yachts, military vessels, or exploratory vessels.
MgT hasn't changed that, it just doesn't discuss that how that is done.
MgT1 Core, p137:
Mortgage or Debts: If the crew are paying off debts on their spacecraft, then these debts must be paid each month. The standard terms for a ship mortgage is paying 1/240th of the cash price each month for 480 months (40 years). In effect, interest and bank financing costs a simple 120% of the final cost of the ship, and the total financed price equals 220% of the cash purchase price.
Note the identical part that presumes a 20% down payment, otherwise the maths are off.
The mustering out benefit covers the down payment, otherwise why would you need the benefit? If there is no down payment, just go buy a new ship on a mortgage?
If the bank transferred the mortgage based on your 40-year economic model then it would probably transfer what is left of the original payment plan i.e. 15 years at the standard mortgage rate of MCr51/240 per month since it doesn't care where it gets the money.
Yes, that is my assumption, but based on the original 20% = MCr 10.2 down payment, so 15/40 × MCr 10.2 = MCr 3.8.
An annuity (fixed payment) mortgage does not work the way you seem to expect. In the beginning most of the payment is interest and only a small amortisation is made. As the decades pass, the loan is slightly lower and interest decreases, increasing the amount of amortisation. At the end of the mortgage very little interest is due and nearly the full payment amortises the loan. So, after half the mortgage period, much more than half the principal of the loan remains.
By your model a 40-year old ship is worthless.
Not worthless, but worn enough to be not worth the upkeep for regular shipping lines. Worth something for someone that does not mind spending a month here and there in the shipyard...
Why would a shipping line select a 40 year mortgage, if it could select a much cheaper 100 year mortgage?
The entire economic model for shipping prices builds on the 40 year mortgage, not 200 year old un-mortgaged ships.
A ship over 250 years old is still worth 60% of it's purchase price (but will have so many quirks it might have a very different spec from what it originally had.
Sure, if you refit it often enough...
Note that one of the most common quirks is "Double maintenance costs".
But, yes, my case is built entirely on assumptions, as is any other model, as this is not detailed in any version of Traveller.
I find centuries old ships, without any mechanical problems or stains on the carpet, unlikely...
USS Constitution is the exception, not the rule, and a lot of money is spent keeping it afloat. It has no economic value, but immense historical value.