So, it seems there may be some rule that's not well applied, or there's no rule but some UK Border officers think it would be a good idea if there was one(and I suppose they do witness many incidents we don't..), so they act as if there is such a rule.
But whether it's a rule, or just sensible advice, it's not well communicated.
I do think that insisting that a strapping boy (or girl) of 16 or 17 is met at the airport is over the top though.