Is age bad for solid state amps?

at least one manufacturer recommends replacing capacitors every 10 years or so. Many mfg's though say don't bother til they break. Most other parts are good for nearly ever I believe, except maybe panel lamps, or electro-mechanical components such as switches.

yes yes and yes..caps go bad transitors die a slow death and all other parts including boards have there own life cycle.Just like us.

Yes, but probably not a worry for the age of the amps your considering, and they usually can be refurbished, especially with an established brand like ARC.

Well, my Nakamichi deck is 30 years old, I bought it 10 years ago and heard no difference until I had it cleaned and aligned. Electronics seems to be just fine. Also, I bought Audiolab integrated new 12 years ago and again - no difference.
So it depends, I guess.