Microsoft 365 office

The model year of an iMac is available at Apple menu > About This Mac.

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to get old versions of macOS - Apple Support

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to get old versions of macOS - Apple Support

A mid 2014 and late 2014 can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to get old versions of macOS - Apple Support

A late 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support

Microsoft follows Apple's three-year sliding window of operating system support and right now, Microsoft will only support installing Microsoft 365 or Office for Mac 2019/2021 for Mac on macOS Catalina and later. They have also disabled the license servers for their retired Office suites, so these cannot be freshly installed on older Mac operating systems.

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

And thanks to VikingOSX…

Let's not overlook SoftMaker's Office 2021 suite which has familiar Word and Excel UI design, and opens these documents in their native formats.