I like my linux and would not go back to Windows, regardless what the ads say! But I am still on Linux Mint 18, should I update to Linux 20?
Mint 18 is end of life. See the release cycle as linked.
If you are going to update Mint, I would suggest 20.01 which will be supported until Aprl 2025.
I don’t know if there is a direct upgrade path between versions but before moving versions, always best to make sure you have appropriate backups of important stuff and some of the applications you use in 18 may not work in v20 although there may be work-arounds with PPAs.
You can always ask questions about Mint in Stack Exchange and research questions about the upgrade in this resource.
I use ubuntu 20.04.2 LTS and have MATE 21.04 on my scratch PC and both seem to perform well,