Don't Install Yet If Are Using an Old Mac
The macOS Catalina update is arriving on Mcs as old as 2012, which is great for long term support, but if you are at the edge of this compatibility, you may want to hold off on installing the update. Let other users try out macOS Catalina on an old MacBook Air or Mac Mini to see how it performs.
This will let you decide if the performance is fine, or if there are issues that will prevent you from reliably using your Mac. Check the MacRumors forums and Apple Discussion forums to hear from users with similar devices.
You are able to downgrade to macOS Mojave, so you can test it out, but you'll want to make sure you have a good backup and that you have time to downgrade if macOS Catalina isn't what you expected.