After near on forty years in the newsagency trade I can state very surely that there is no law about selling newspapers to minors; there never has been and there is not know.
This is just Tesco yet again becoming the unelected moral guardian for the nation because they think they own us all! As with the recent cases of adults being stopped from buying alcohol when accompanied by children, Tesco staff have yet again exceeded their role as purely a retailer.
I have now stopped using Tesco and destroyed my Clubcard in protest.

P.S. The law about not being able to sell certain products on a Sunday, such as Bible's went out with the introduction of the current Sunday trading laws whereby any store in excess of 3,000 sq. ft. can only trade for six hours on a Sunday, except on Christmas Day and Easter Sunday when they must not open at all. All other smaller shops can trade freely, and no shop has any restrictions on what it can sell on the Sabbath.