Good questions.
WRT my mum: Her transition from Windows to GNU/Linux happened in 2010 when we bought her a new Novatech (a rebadged Clevo) laptop. Her main uses are email and web. She was already using Firefox and Thunderbird on Window XP. So much of her move went almost unnoticed. Libreoffice writer was close enough to the old Word 97 she was using - before MS decided to use that horrid "ribbon" thing. So all-in all, her transition was pretty smooth.
She even did her own software updates until very recently - they are generally pretty smooth and straightforward too. However, these days I generally do them for her remotely over ssh (secure shell), same time I do my paying customers and our own remote servers. I usually perform an incremental off-site backup of her home directory at the same time, using rsync over ssh. Debian's OpenSSH implementation is excellent and Debian's Security Team is very quick to address any security issues that may arise.
WRT how old can the machine be? I'd say pretty old. I just did an upgrade on an 2010 vintage 17" Vaio laptop running Win 7, for my cousin up north. It was quite a high spec machine at the time of purchase. It's an early i5 c/w 6GiB RAM. It was pretty straightforward too...
- Replaced its electromechanical HD with a 1TB SSD and put her old drive in a cheapie (£10 off eBay) SATA enclosure.
- Installed Debian 12 (aka "stable") from USB stick onto the new SSD drive.
- Installed all the app I think she needs. This can be done a single, no-questions-asked, terminal command (rather than squillions of tedious "setup.exe" executions).
- Hooked up the old drive via USB and copied across her data.
- Set up her email accounts in Thunderbird (granted, moving legacy locally-stored POP3 emails from MS Outlook was a bit of a pain but it's doable via a spare/throwaway IMAP mail box).
- Imported all her old wallpapers and other bits and bobs. I wanted the machine to look and feel fairly similar to her old set up - just faster. O/P wouldn't need to worry about this.
- Wrote a brief set of instructions and links in Zim wiki. I also set up sample pages for her own notes, mostly copy'n'paste from my own Zim notes. O/P probably wouldn't do this either.
- Finally, I formatted and encrypted the old drive, which she now uses to back up her home directory.
The performance improvement was stunning. About 15 seconds from "post" to GUI login. And 3 seconds from GUI login to usable GUI. (Took around 4-5 minutes in MS Windows). Thunderbird loads up in seconds (took several minutes in MS Outlook). Plays MP4 & MKV video in VLC and SMPlayer in full HD. LibreOffice Write, Calc and Impress open all her MS Office documents without issue. And Okular has proven a much faster and more useful document viewer than Acrobat reader. Of course, some of the performance improvement is due to the SSD. As it happens, Debian and its derivatives love SSD. But not having to run any antifungus software also helps. Debian is particularly adept at adapting to the hardware upon which it runs. In fact Debian is arguably the most scaleable OS currently available. We run it on everything from a tiny Raspberry Pi, right up to multi-user web and mail servers.
FWIW I'm on a Debian machine right now - a 2020 i5-series10 Clevo with 32GiB RAM on a 2TB Samsung SSD, running Debian "testing" branch (next version on from "stable"), fully loaded, with full disk encryption. Goes like the proverbial clappers! And for the record, I'm really looking forward to all those lovely high-powered non-TPM2.0 machines hitting the market, dirt cheap! :-)