2009 iMac (21.5display) ver10.13.6 High Sierra. Would like to use computer withA new mac mini m2 or even an older mini. I have a mini display port on my iMac.
Mini DisplayPort: Connect to a Mini DisplayPort. Use an adapter to connect to a DVI, HDMI, or VGA display.
iMac, OS X 10.11
Note that the Mac that you're connecting to your iMac must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed. So you'll need Mac mini (2018), Mac mini (Late 2014), Mac mini (Late 2012), Mac mini (Mid 2011).
See Use your iMac as a display with target display mode - Apple Support or Identify your Mac mini model - Apple Support. Bottom line is that you can't use an Apple Silicon Mac mini. Any other previous version should work.
Note that the Mac that you're connecting to your iMac must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed. So you'll need Mac mini (2018), Mac mini (Late 2014), Mac mini (Late 2012), Mac mini (Mid 2011).
See Use your iMac as a display with target display mode - Apple Support or Identify your Mac mini model - Apple Support. Bottom line is that you can't use an Apple Silicon Mac mini. Any other previous version should work.
The 21.5" 2009 iMac can't be used as a display with any Mac of any age. Only the 24" or 27" versions can.
Use your iMac as a display with target display mode - Apple Support
connect 2009 iMac 10.3.6 (21.5" display)with mac mini
