TO reiterate what has been said, condition has a lot more importance than years in service.
One thing to bear in mind is that aging aircraft have "experience" in service. That means that information as to their shortcomings has had time to be analysed, and circulated throughout the industry. Once a shortcoming is reported by Service Difficulty Report (SDR), the regulatory agency will circulate that information, and prudent maintainers will include extra scrutiny on that new "trouble spot". If more are detected, and reported, more serious action (AD) could result.
It is your responsibilty as the pilot to be satisfied within your skill set that the aircraft appears airworthy. I agree that an aircraft which looks chronically poorly maintained may be worth a pass.
As for mandatory retirement, strucutral components of large aircraft, and very certainly helicopters tend towards this means of assuring airworthiness. If, however, a person wishes to buy an aircraft, and retire it, they are certainly welcomed to do that. It would seem that most owners would rather maintain and fly it though.
In 2005, I undertook a major structural modification on a 1979 Cessna 207, with 19,800 airframe hours, with a lot of that Grand Canyon tours. This well maintained aircraft was in excellent condition (I got to know it's structure very well over six months of having it in pieces). I found one small crack in primary structure, which would seem to have been there for a long time, A recurring inspection program was developed, and there was never a need to replace or repair, other than stop drilling. It's poor paint, and faded windows, which appeared to indicate a tired aircraft, were the only negatives about it.
Appearance can be an indication of conditon, but not always. If you're really interested, have a good read through the last few years of maintenance entries, if they are available to you in a logbook. Those are a very good indication of the conditon of an aircraft.
And how old were those three aircraft which over flew the Royal wedding?