The best way to tell the age of a flywheel is to cut it in half and count the rings.
Just kidding.. I have no idea how you'd tell the age of a flywheel unless there were a part number on it.. but my guess would be that the flywheel is probably the same year as either the tranny or the engine.
When it comes to flywheel thickness, i'd say it's probably pretty important since the amount of "grab" you have is based on how tight the pressure plate pushes the clutch plate against the flywheel.
If the flywheel were significantly "thinned" then it seems like the pressure plate would have to press further in order to provide the same amount of "grab".
Now this is all just a guess based on my limited experience working with a clutch and flywheel in my '46.. and I know that plenty of folks have their flywheels ground down a bit when they have lots of rust on them, or when they let the clutch plate wear down to the rivits and it starts to groove the pressure plate and flywheel.
I'm sure there is a minimum amount of thickness the flywheel needs to be in order to work.. that thickness would depend largely not on the engine year, but on the kind of tranny you have behind it, i'd guess.
