Old Startup disk (Lion) in a new computer

To be a bit more specific., when you have a disk with either an installer or an OS on it, the file is for a specific OS such as High Sierra or Catalina. It is not for any specific year and model of computer per se. Then, depending on the OS, certain models of a computer can run them. In the case of a late 2009 iMac, it can run High Sierra or several earlier operating systems. Every operating system is only supported by Apple so long. As new ones are released, older ones become non-supported. High Sierra is one of them, although it is a fine OS and will function well indefinitely as long as the computer functions. Keep in mind that a 2009 computer is a bit old and could fail at any time. As to what operating systems can run on which year and model of Mac, that information may be obtained on Apple's support web site, if you need to know. I hope this answers your questions sufficiently.

Mac OS X Lion was introduced if memory serves me correctly in 2012.

Any Intel mac prior to that should be able to run Mac OS X Lion you

can research all this information from this website, https://everymac.com

In may not be as simple a case to install the old Lion hard drive in the mac

you wish to use as the boot refi for each mac/ OS could be different meaning

the 'newer' mac may not be able to boot to the old disk from the 2007 mac

as the efi protocols on the logic boards could be different.

Keep the old hard drive in the 2007 mac, connect it by ethernet cable to the

'newer' mac and transfer the OS and all the data using the Migration Assistant

app from the Applications> Utilities folder.

No. No Mac will allow you to install or run an older operating system than the one it originally came with.

You can always try it and see if it loads.