I'm a huge Linux fan but "it just works" is not a phrase I'd use in the same paragraph as Linux. Try explaining to your grandma that she needs to open a terminal and run "sudo apt update && sudo apt upgrade"

It seems that you haven't used it in a long time then. You don't need to go the terminal to update the system. Mostly you get a message that there are updates for the system. You click it, click update all and your done.