As far as "old" goes, you'd have to consider something 10 years old to fall into that category. If you're talking about a 2-year-old sub, then I think that baby is just starting to "sing" and is hitting its stride.
Amplifiers will not "fade" - more than likely, if there's an issue with an amp, it will be pretty obvious.
If you're considering a used sub, go for it - provided it was made within the last 2-3 years. Anything before that might be considered "ancient" not because of the age of the sub, but rather, the advancements that have been made (e.g more powerful amps, better drivers, etc.)
What Wayne said. 2-3 years old is safe. If you start to approach 10 years old, I'd start being nervous about potentially rotted surround material (depends on how they are constructed). I have bought a couple of used subs before and had no problems. Biggest thing is to carefully inspect the cone itself for any damage.