Alcohol is harmful, and you want to prevent minors from obtaining it without parental supervision. Do you pass a law requiring every car to log the age of every occupant in case the driver drives to an establishment that sells alcohol? No, that's stupid. You require the person providing the alcohol to check age only when they are about to hand over the alcohol. Until someone actually attempt to access alcohol, they should not be asked their age.
Now exchange "car" for "OS" and "alcohol" for "age-sensitive content"
Now exchange "car" for "OS" and "alcohol" for "age-sensitive content"