[user104658]

The issue is that you'll always get some extreme outliers - I knew a guy at school who had long bushy hair and a full (and I mean FULL, like bloody Hagrid) beard at 17. There's NO WAY I would ever have age-checked him in a shop... but he wouldn't even be just under 25 or under 21, he was full on underage.

That's the sort of person that they like to use for the actual council/government ID compliance checks, too. Authorities can use people who genuinely shouldn't be making the purchase (usually 16 - 18 year olds). Internal ID compliance tests (by the employer) can only use "legal but under the testing age" people, so 18 - 20 year olds on an 18+ legal/Think-21 sale for example.