YES, aged propane tanks need to be replaced.
Or, conversely, even if they could be repaired, it's so labor-intensive that replacement simply makes better financial sense, especially given that reputable outlets will not fill older tanks.
FOCUS PRIMARILY ON THE CONDITION OF THE FITTING THREADS WHEN MAKING THIS DETERMINATION.
Although the coatings (paint) need to be maintained properly, these tanks are of such heavy gauge steel that it might take longer than any of us have in our remaining lives for them to rust through to the point where the endcaps or sidewalls became structurally compromised.
However, the threaded line ports are fragile (anyone who has studied materials science could tell you why, but I'll skip the explanation for brevity). If the THREADS rust out, then there's no way to stop leaks aside from either re-threading (= a repair) or replacing the tank.
I replaced our OEM Manchester #68162 tank (a model that is no longer mass-produced) with a compatible Manchester #6813 tank because of rusted threads. You can read about that adventure here:
REPLACING A LEAKING LP TANK ON AN AIRSTREAM INTERSTATE
The Manchester #6813 can be purchased online for about $400, and then there's the installation charge.
I also replaced all of our propane flex lines, and the details on that retrofit are described here:
REPLACING THE LP FLEX LINES IN AN AIRSTREAM INTERSTATE
I did this because I saw too many owners developing potentially catastrophic propane leaks, and because the flex lines installed on Class B and other RV models have a manufacturer-stated service life of
just 5 years.
The fact that this 5-year trigger is apparently not formally disclosed to RV buyers has been the subject of many forum threads. It has inspired a lot of ostrich maneuvering on the part of owners who cling to the belief that, if they haven't been told to do it, it must not be necessary.
Of course, there is abundant evidence to the contrary. For example,
in this recent sub-thread, a long-time owner concedes that, even though she knew that her lines needed to be replaced due to age, she did not do it, and lo and behold, her inaction resulted in an uncontrolled propane leak. That is not a gamble that I personally would ever take - I value my life more than the few hundred bucks it takes to have new lines fabricated.