Is it possible to revert to an older OS on a Mac

[Jurek]

As above.
My 2015 iMac really isn't happy ever since I installed Monterey.
Very slow to start up. Often requires re-boots -which it never did when running previous OS.
So, is it  possible to revert to an older OS, and how would I go about it?

[ian]

If you have Time Machine, hold command-R as you start to open the recovery centre, and select a TM backup from before the upgrade.

[CommuteTooFar]

When I used Macs I had a nice Mac Pro.  When a new OS appeared I added a new hard disc/partition. I had 10.5 to 10.10. Then Apple commercially decided that my Mac Pro was too old and 10.11 said no to installation. I probably could of tricked the installer but it was not worth the effort. I left the version checking to the test engineers so any version faults with our software would be one step down the line.

By this time my company had stopped supporting the older OSes because the application programmers used libraries that did not. My device driver part of the project continued to support all our customers. The first customer dropped their requirement for power pc and no one else ever wanted it so that was stripped from the build. I had to stop using XCode editor because Apple's drive for over simplifying everything did not support more than one target. I needed four to bridge that many versions. I rolled my own XCode project file and used SlickEdit to edit it. (That broke SlickEdit which they fixed within a week). The command line version of xcode fully understood my funny project file and built whatever I told it to. 

[citoyen]

I did this recently. It was a bit of a faff but doable. Can’t remember the exact procedure though. You have to download the old OS (get it from the App Store) then restart your machine in a certain way (like ian says) to allow it to start the installation procedure.

I did a complete rebuild but I think it’s possible to just install the OS and keep your files intact - as long as you have enough reserve disk space.

[Jurek]

If you have Time Machine, hold command-R as you start to open the recovery centre, and select a TM backup from before the upgrade.

Thanks for this.
I've sent it on its journey into the past.
So far I've had nearly an hour of 'Preparing to restore "The Drive Business"' (Catchy nomenclature bestowed upon the SSD I installed).
I think this may take a while.

[StuAff]

Macs have always been able to run any OS version with support for that machine- so you can revert to any version starting with the one it shipped with. In the case of this ancient MacBook (2009) it's version 10.5.6- I'm running El Capitan, the last officially supported version. The elder Mac Pro (2006)- running Lion, can go back to Tiger 10.4.4. Newer MP (2010)- running Mojave (& Big Sur on hacked install, haven't tried Monterey yet, but it'll work) can go back to Snow Leopard.