I'll let the other aforementioned members address the vintage part of the question as they have tons more experience. However one of my hobbies is purchasing and refurbishing older regulators (mostly ScubaPro.) Plenty of them have sat for years-sometimes put up wet and still work fine. Sitting for a year unused is far more detrimental to a regulator's health than a year's worth of constant use. I'm lucky when I find a sample that has a sticker indicating the "next service due date" is often 10-15 years ago. Exceptions are regulators that had kits installed with polyurethane seals. Unfortunately these seals turn hard and crispy sitting around for long periods of time.
To the OP's more specific points. I have gone over 5 years between servicing one of the sets (MK10/Balanced Adjustable) in my stable with no degradation of performance noted. Yes, I have a full blown flow bench available for checking. For those who do not have such a luxury I invite to review the checklist at the top of this forum.
Regulator Inspection and Checklist (Rev-8)
Do I recommend others go years between service? Certainly not. But the checklist helps regulator owner to make an informed decision.
