For official purposes, the model year will be what the state has listed on the registration.
A 12 year old fiver that has been well maintained can last many more years. Minor cosmetic issues are to be expected. I'd actually get up on the roof if the owner lets you. You want to make sure there are no neglected areas such as missing or seriously cracked sealants. Your looking for water leaks is a really good idea.
I'd also ask to check out all the systems. Fill the water tank and make sure the pump works and it doesn't leak. Make sure the slide out works correctly. Verify the landing gear is working without any unusual noises. Does the AC work? Do the gray and black tank dump valves work? While it's plugged into shore power, use a volt meter to make sure the converter is working.
Keystone products such as the Cougar have been known to have some quality issues, but as long as the structure and roof are good, you can work around other problems.