About box age counts from the install
A change merged into quattro counts the About box's OS Age from the install. The pull request says that since 4.5 the root is born when the ISO was built, so the age was too old by the ISO's age.
The pull request says the age was negative when the installed machine's clock ran behind the build host's, and that a fresh install showed "-10 days".
The change reads the birth time of /var/log instead. That path is the log subvolume both installers create during the install, the older installer before 4.5 and the image since. On a system set up by hand, the age still dates from pacstrap, as the root does.
The setting is the system one in /etc/fastfetch, so an installed machine picks it up with the package. There is no migration.