Within a Venue's Settings Page in the Purple Portal Systems* you can find something called Today's Unlock Code
This is to allow staff to validate an underage client (i.e. under 13) wanting to be allowed on the WiFi (with their parent's consent).
The default age restriction is 13, but this can be set on Onboarding > Access journeys > on the Options page (when editing your Access Journey Template).
*this option is not available on https://region3.purpleportal.net (2019-09-01)