Could someone clarify for a linux newbie like me... In practical terms, what does this mean? I'm on Debian so presumably Debian will eventually pick this update, and then what? When I upgrade my system I'll get a prompt asking for my date of birth?

hello,

as always: imho. (!)

idk the exact procedure which will apply to enter the birth-date on such a system, but if other comments are correct: just enter what you want!

there will be no real possibility to tie this to anything "legal" / to "enforce" any "official" check of lets say your passport or other governmental id.

and if in my personal opinion (!) the pretty crazy guy behind the systemd-project tries to introduce/enforce such a thing ...

then i think it'll be time to either fork the project or look at systemd-free linux distributions like devuan ~ a systemd-free fork of debian :)

* https://devuan.org

just my 0.02€