In effect we have that system for buying things legally prohibited to under 18s. "If you are lucky enough to look under 25,... "
Consider your own words "legally prohibited" and think again - not remotely the same as this.
Point was the legal prohibition applies to under 18s, the commercial interpretation is for under 25s, or rather those who appear as if they might be under 25 – not 18 – to prove they are over 18.