You need your serial number. You can get this by going to the Apple Menu, then selecting “About this Mac.” Once you have the About this Mac window open, click on the part where it has the Version X.X and your MacBook’s serial number should replace the version number.
Once you have the serial number, enter it in the box on this link (https://selfsolve.apple.com/agreementWarrantyDynamic.do ). Make sure to check and be certain that the link is actually Apple on any site that asks you enter your serial number. This will show you your Mac’s current warranty status and eligibility for extended warranty, and should give you information about when it was built.
Even without your serial number you can also use. http://www.everymac.com to determine age based on other characteristics such as case type, CPU, series, etc.