there is a list that the ATF puts out, of what guns are considered curio and relic. some guns even made in the 1980s such as the CZ82 are on the C&R list.
say you've got an old ww2 mauser or lee enfield. that's C&R, but if someone sporterized it, even if the work was done in the 1950s or 1960s, its not considered C&R anymore because it has been updated and modernized.
as far as age, i'm pretty sure anything 1897 and older isn't regulated by the ATF becasue its an antique. it may shoot, but its not really considered a firearm as far as the ATF is concerend.
edit: this is a great resource.
http://www.surplusrifle.com/shooting200 ... /index.aspsurplusrifle has provided me with hours of reading enjoyment on the topic of milsurp.
C&R is good because if you are a C&R holder, you can get guns shipped directly to you, not the pawn shop around the corner with an 03 FFL.