The lifespan is comparable to a cell phone (as in you'll probably break it mechanically before you burn up the CPU). If you're overclocking it and running it hard without cooling, I'd expect it to die after a year. I've never had one break from a cooling problem or any mysterious bricking that might have been cooling. Even when stress testing, they throttle back on their own, and are idiot proof as far as heat goes. The thing that will frustrate you far more is the degraded performance.
And no, if you aren't experiencing throttling, then cooling them isn't giving you any advantages. Marginally prolonging the life of a $35 computer with a $5 fan isn't cost effective- consider that when you have to buy a new one in a few years you will be able to buy more performance for the money or get the same performance for much less. Think about the upgrade from the original Pi to the Pi3, or the cost drop to the Pi zero.