When will this become available to the iMac 21.5-inch late 2012?

Does the >>  Computer Qualify for the Monterey upgrade  and if not listed - this computer does not qualify

The Best this computer can run is the present Catalina and no further.

macOS Catalina is compatible with these computers

macOS Big Sur is compatible with these computers

Time to consider a new or newer computer to get to Monterey.

Apple does sell Refurbished computer that are just as good a new but at a reduced price. Have used this several time and has always been satisfied

Then there is the Trade In for a New Computer where the value of the Old is used against the cost of a New. That too have used and has worked well.

macOS Monterey is compatible with these computers - https://support.apple.com/en-us/HT212551

It won't.

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