Because I am a compulsive spender I tend not to keep equipment as long as I should.
That being said the oldest amp I owned was a set of Manley Neo Classic 250s. I bought them used when they were 7 years old.
I traded them in for a set of Class D monoblocks 3 years later because I couldn't stand the heat and the tube maintenance.
At 10 years old they ran like champions and had not one issue.