This is stupid. The age should be in the passwd gecos field or somewhere else in the user's config directory. Not in systemd. Unix-ike systems are multiuser. Now I wonder what age to put in the root, adm or games accounts.
Systemd actually manages /etc/passwd, /etc/group, /etc/shadow, etc. using a json database called userdb. Adding a field to systemd's userdb is how you add a gecos field.
Look up systemd-userdb (the systemd component that added this field). Like the sibling comment said, this is basically equivalent to adding a GECOS field. A totally optional field.
