At about 20 years old some amps will have capacitors that will need replacing, but that usually isn't too expensive or difficult. Tube amps obviously might need retubing at almost any age, but even if you just look at inflation and depreciation, the math quickly tells you that older amps can offer tremendous value. For the same money as new "entry level" amps you can be looking at older near SOTA amps that will perform at a much higher level. Cheers,
Spencer
Spencer