Providing it still has the original barrel, you need the barrel code(s).
They are located on the left side, just ahead of the receiver. They will be letters not numbers.
Then go here:
There is a good thread on the RemSociety forum about the ages of member's 721/722rifles here:
Remington Society - View topic - 721 722 725 dates of mfg
Be careful with those old 721s and 722s. The extractors are no longer available except for the belted mag bolts. A good 'smith can still fix you up though, if you break one.