does overclocking a CPU always decrease its lifespan?

most decent CPUs are robust enough in terms of design to overclock without damage.

With adequate cooling it should be cool, as long as you don't over do it.

It is impossible to say for certain, given the variables involved between different processors and different overclock settings / cooling. With decent cooling and factory settings, the cpu should last for more than 5 years, possibly even 15.

However, the real question is does overclocking decrease the cpus effective lifespan, given constant improvements in tech, expecially considering that an overclocked cpu might be usable for a year longer than a factory one given the slowness of the factory setting.

From my understanding, as long as you have good cooling and don't overdo voltages, the cpu should last long enough to be obsolete (3-5 years at least)

The official answer is yes but it's marginal with the right cooling and a reasonable overclock. I personally have had several computers down the years and have never known a CPU fail yet. I doubt if you are likely to reduce it's life so much that you actually see it fail before you are already upgrading again.

As long as it is within 10 degrees Celsius of it's safety temp. and you have some quality thermal compound applied, and a decent heat sink it is perfectly fine just make sure to check those temps. Also next time let us know exactly what processor you are attempting to overclock so we can give you a better answer.

If it's a stable overclock, there shouldn't be much of a difference

As far as I know, yes. But it could be so marginal it doesnt matter. Anyways, computer upgrades come out so frequently you are doubtful to use a piece of hardware till it goes bad.