When is used used up?

Is there a rule of thumb that says the savings of buying used equpment is offset by the age of said equipment? Surely, aging negatively effects circuits -- welds weaken, capacitors lose capacity, transistors tire, wiring oxidizes, etc. My first amp in 2000 was a Mcintosh 250 in A- cosmetic condition. When I replaced it five years later with an HK PA2400 (used), my system took on new life (more than the increase in wattage).
Seems like when a component is old, it's old, be it sacred cow or not.