I use a simple cobbled together house rule method of aging for ships. It's not real financing, just simple gaming. Basically:
Each full 10 year block of use imposes a DM-1 on the chance of breakdowns. Normally roll once a month, 13+ per system to have a breakdown. Other practices may impose other DMs. Missing engineers, skipped maintenance, etc. A 40 year old ship with no overhauls has a DM-4 on the breakdown check. This breakdown DM may be negated by performing an overhaul. Overhauls require a Class A starport, 1 month, and 0.005 x ship price.
Each 10 year block or fraction of is 10% depreciation of ship value. So the ship is only worth 90% as soon as it leaves the shipyard. A 40 year old ship with its (first) mortgage paid off in full is only worth 50% the new value. This revaluation is used to factor mortgages on ships that aren't new, but mortgages will only run for a max of 40 years or the remaining value life of the ship if less. A 60 year old ship would have a value of 30% and could only get a mortgage period of 30 years. A 90 year old ship has no mortgageable value. It may still have resale value and a long life ahead of it though.
The resale value of a ship is the above, adjusted for care and maintenance. Each 10 years of wear (breakdown DMs) offset by an overhaul subtracts 5% of the above devaluation. The 40 year old ship with no overhauls would have a resale value of 50% new price. The same 40 year old ship with regular overhauls every 10 years would have a resale value of 70% new price. A 90 year old ship would have no mortgageable value but if well cared for (9 full overhauls) would still have a resale value of 45% new price.
It is even possible for ships to continue flying when nobody would offer anything for it if put up for sale. There is no practical life end expectancy for a ship with regular overhauls. It is essentially entirely rebuilt every 200 years or so, in bits and pieces over the years. It's not really the original ship of course but a collection of replacement parts. Maybe with a few odd bits that are still original. And it won't have the same marketable value as a newer ship. A 180 year old ship would have long outlived its mortgageable value (by double, 90 years), and though with regular overhauls it would be as reliable as the day it first flew (DM -0), it would have no resale value (100% - 190% for age + 90% for overhauls = 0%).
But that's Book Value. You never know, someone might still offer you something for it. But would you take it? Everyone has their price...
