Comments invited: How/why do CD lasers fail?

I'm considering various CD transports at present, and these are older models(that's the only way I can afford one!). Thus I need to make sure of availability of lasers/mechanisms to avoid having an expensive paperweight. Whilst I understand the mechanical failure modes of transports, I'm wondering what causes laser failure? Is it purely a function of age, or of use? i.e. if I had a laser mechanism that was 10 years old but unused (NOS), would the laser still be good for a 'normal' life?

Do laser units have a shelf life?

....& whilst on the subject, do solid state components degrade with time or use (probably both, I suspect)? Would an unused 10 year old resistor, for example, be as good as a new one?

Thank you