With a car, i'd say anything greater than 10 years is approaching the end of its useful life and above 15 years we're approaching relic. also with a car the differences between a 5 yr old car and a 10 yr old car are pretty noticeable both interior and exterior.

What about with commercial jets? i read AA flies more jets that are over 25 years old than any other airline (or so I recall). Is that considered really old? Anyone know the average age before retirement for the major airlines?

I myself can't really tell the difference between a new jet and an old jet? What about you guys? Especially with how they replace/fix the interiors every so often, does the age of a plane really matter (within the same model)? Of course its annoying if the carpet looks all faded and the seats are ripped, but those types of things can be fixed/replaced.