My Macbook Air is to old to use the newer versions of Tor. Is it in this case still safe to use the older Tor version, or do I really have to buy a new computer to use Tor safely?
No, it is not safe to use old versions of Tor Browser or any other internet connected software.
This is explicitly what it says in the out-of-date version warnings so I'm not sure why you're asking here again as if somehow the warnings were just a joke or something.
It's a shame that your operating system vendor likes to price gouge so much, you might be able to opt to run an operating system like Tails to get around the restrictions put in place by Apple.