Going by what some members on here discount as classic (aircraft that are still in service or only recently out of production), I get the feeling the majority
do just determine itby age... it is probably the simplest way. And, as has been said, as with the Austin with the sqaure steering wheel, some people have some bizarre ideas of what is nice and classic... theres a disturbing number of people on here who think that ugly and ungainly Beverlies and Belslows are classics!
For me, it has to be any one
or combination of the following:
Successful (in terms of sales and number of operations)
Aesthetically beautiful (easily achieved by most British aircraft

Reliable or sucessful in achieving the technological goals set out in its design
Be notable for some reason (e.g. Harrier - VTOL, Trident - Autoland, Concorde - Mach 2 airliner, Lightning - vertical climb and first Mach 2 interceptor, 146 - STOL and super-quiet)
So if I were to run this as a test:
Sea Harrier:
Aesthetically striking
Sold to two nations, served for over 25 years, sucessful in Falklands and Balkans conflicts
Succeeded in providing a weapons platform from a very much cheaper carrier
Notable for vertical takeoff, landing capabilities and excellent radar and air-2-air weaponry
Shorts Belfast
Fat and ugly
Sold 10 or was it 11? Served for about 10 years with the RAF and 1 or 2 made it to commercial operators
Rather slow and outperformed by the Lockheed Hercules
Notable for being large, ugly and slow
I rest my case.
