How do you know when to recap a unit ? Is there an age when it should be automatic ?
It would be nice to have some really hard-assed empirical stuff on this subject, but perhaps the variables make it impossible. Each manufacturer does publish data, and you can find it on the spec sheets. Still, it may not help us with your question.
The core of the issue is that the wet paste that makes up the aluminium oxide dielectric compound tends to dry out over time. Heat that approaches or exceeds the design factor (e.g. 85 or 105 degrees) and time are commensurate factors in determining the life of the dielectric. In addition, as rulerboyz pointed out, inactivity is also a factor contributing to cap deterioration. Old caps can be "reformed" after inactivity, and a search will reveal some articles on how to do it. The process involves restoring the thickness of the dielectric layer by applying gradually increased voltage from an initially low pressure.
As for just the 'when' question: sorry — can't give you a reference ... but one opinion that I thought was authoritative when I read it said that in general caps begin to deteriorate after 5 to 6 years. I have also read a number of times (from various angles) that caps are perishables and have a shelf life. This makes even NOS selloffs less than ideal purchases.
Having said that, here is a story that echoes something of other comments you have just read: Three years ago on a visit to Canada, I dug out my 1970 AR integ. amp with the intention of packing it back to Japan. I cleaned it and took it to an old friend's home where we ran it in a shoot out against a recent model NAD. The speakers were sealed boxes that needed some power. Now, I half expected the AR to sputter, shriek and smoke. But we put on some big rock as a source and drove the living daylights out of both amps. OK, the NAD is no super high end machine, but I have owned one and I have a lot of respect for them. And this one was nearly new. On the other hand, the AR has run countless thousands of hours over 18 years, often hot as hell. Still, it ran an even race with the NAD, albeit with a tad more grain. (Amazing considering the developments in a number of areas.) And it had not even been powered up for almost ten years. I expected either the neighbours or the police to start beating on the door, but the old amp never faltered.
So what to say?: Lefty, YamahaB-2, rulerboyz, crooner and Braddog100 — all have given some advice. Some of it is even contradictory and yet all of it can be considered sound advice. This shows that the question is open to discretion.
To be sure, caps DO deteriorate over time. Bulging and leaking caps are a disaster waiting to happen. Old and tired caps in capacitor coupled amps are going to rob some sound. Optimum performance of ALL the components results in optimum sound. Take these things together and suit your pocketbook and predilection. For myself, I have decided that all my old gear could use recapping whenever I get around to it — including the old AR. And who knows? It may have a lot more to offer than when I last heard it. The shootout demonstrated that it was still great for hard rock, but I think it could play the baroque viola de gamba better at civilised volume if everything was freshened up. And there is only one way to find out :yes: