Update
I have not updated my Macbook Air since 2016, is there a way to get it updated to 2021?
MacBook Air 13″, OS X 10.11
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The model year of a MacBook Air is available at Apple menu > About This Mac.
A late 2010 through mid-2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version. See: https://support.apple.com/en-us/HT211683
A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683
A mid 2013 and early 2014 can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT211683
An early 2015 or later can be upgraded to the current version of Monterey. See: https://support.apple.com/en-us/HT212735