well aging is relative, it mainly comes down to how well they where looked after, and how good they where to begin with, and more often than not you hear of a story of someone reclaiming an old scope from a backyard shed of hell for the thing, doing a good job cleaning it and calibrate it and done,
biggest issue that you may face is postage, not the repair, because the CRT tubes in older scopes can be very fragile in comparison to modern day fragile goods handling, (e.g. if you want something to survive auspost, wrap it to survive being thrown out of the second story window, or whatever different regions post may be like)
if that's not a concern, then you also want to make sure you can get documentation, both user and service on the scope on hand, say on BAMA (boat anchor manual archive) or from various groups or sites, as you want to know what your getting can be serviced easily, ironically, this is generally easier to get for older scopes,
as for how old is too old, well that depends on what your getting it for, and what functionality your need,
far too many tech-heads go chasing prized tek curve tracers because back then they where designed in a way that is more useful than today's products can compete with, similar things can happen with scopes, there are some odd and rare models of various makers, designed to fill niche's that never really took off, but on there own are great products, including almost handheld dual channel CRO's, to 4 channel separate time base monsters,