How do I use serial # to identify iMac age?

"About this Mac..." works if yours is running os 10.7 or higher.

If that does not help, enter your Mac's serial number into this Apple support page:

Check Your Service and Support Coverage - Apple Support

The return will look something like this:

Sometimes the Apple lookup tool does not look back more than a few years, If the site fails to find a result, this third-party site does the same thing but goes farther back:

https://everymac.com/ultimate-mac-lookup/

you can go to this page: Apple - Support - Technical Specifications. then enter the serial number in the "search" field. it goes at least as far back as 2008 for MacBooks. I'm not sure how far back they go for other Macs. the 2008 MacBook is my oldest Mac that I have to try and do the search for.

you can also click on the description of the product (below the picture) to see all specs for the product.

it also works for iOS devices.

Below the Apple logo is a link "About This Mac". Click on it and the window that opens will have all the information you need with buttons that give access to even more information about your Mac.