The serial number on the machine can be matched to the listing online which will tell you when that serial # was allocated for production.
Some people like to get a machine from their birth year. Others are looking for the oldest one they can find. And some of us just get the first one we see that doesn't need fixed!
As far as I know there are two models of featherweights - the 221 and the 222. Mine is a 221.
I understand the 222 is more rare.