Well, they may not. The rate at which caps age, very much depends on manufacturer of the caps and materials used more than anything else, and usage. Long periods of disuse can also accelerate the problem.
The failure of other components is random on the whole, although with a tendency to fail when new more often than not.
In this case, I was just purely speculating on the likely cause of a loss of gain. If that happens to an amp it generally means there has been a loss of voltage to an area of the amp.
And of course no evidence was provided in this case that this issue, was actually a problem with your amp, and not another piece of equipment in the chain.