My mother used Linux Mint as her last OS just fine. She struggled more with her phone than Linux. Just using browser, mail client and writing the occasional letter you're pretty much OS-agnostic.

However, while that does work, sometimes updates break something (regardless of distribution). Windows mixes shit up which makes the elderly not finding something again but Linux updates may result in the DE not starting for some reason. I moved to Linux 100% myself but I still won't ever recommend it to anyone who isn't fine with tinkering or has access to family tech support.