Illustrator - saving as older version

I am working in Illustrator 2020
How can I send an ai file to a client working in Illustrator 2018 so that she can keep working on it?

Open your ai file, and go to “File” > “Save A Copy…”
Choose the file type you want to save it as (illustrator in your case)
Rename your file so you don’t replace your own file, and save.
When the grey box pops up with options, right on top you can choose what version you want to save it to.
Then just click ok.

Thank you, though my question now is, which one of those versions are compatible with illustrator 2018, they’re listed as 8-9-10-c26…

I would think CS6… but I could possibly be wrong.

I agree with Goldie that CS6 should work. Though the truth is that you can always save as a lower version and if they have a higher version, they can open it…higher version software can open lower version saved files.
So if CS6 doesn’t work, you could always save, for example as version 10. But be aware then some data might be lost when you save down to a lower version.