How old is too old (for SS gear)?

I've personally not been too concerned about the age of used solid state amps and preamps, but recently a few pieces of gear I've purchased have either A) bit the dust or B) starting to make weird noises.

So now, I'm a bit gun-shy about buying used gear much older than a few years.

Do you have a certain age limit when considering the purchase of used gear?

Thanks.