Doesn't that depend on the vehicle and the person?
My MZ is mostly 14 years old although the bottom of the engine is a bit older and I've no idea about the top, it came off e-bay. All this work was done at home, it cost me about 50 hours over last spring. The bikes paper value is something like £300, so depreciation is at worst zero. It's reliable and I can live with the 40 mpg as the cruise speed is up to modern standards. It actually saves me hard cash on the insurance as it brings down a two bike policy. Enviromentally I'd debate that the worst of the damage was done by the East Germans when they made it, but you'd be correct in thinking it isn't exactly green. I'll replace this bike when parts become a problem and i've the time/cash to put a Diesel in a Bullet frame.
The other side of the coin is that BMW R1100R I had. 5 years old, but an unreliable pig due to over complex engine electrics. It was on it's way the third time it died in traffic. Parts weren't that bad, but you can't afford to keep a FI ECU just in case. The value was such it made sense to get shot before it took a huge drop.
If a bike is generally reliable, does what you need it to do and can be kept going without superhuman effort, I'd try and keep it.
Andy