Beer Tesco

Be alcohol served?
My dad and my sister went into Tesco last night and tried to buy some beer and light appears ALCO. The person in the box denied my father (over 18 lol) to be served any drinks when I was with my sister, whose 15. Is this good or bad? "It contradicts this law to purchase alcohol at 18?
if they suspect their being bought by someone under 18, can refuse to sell
They sell Cooper’s Pale Ale in Tesco.