In a nutshell:
Tesco have a ‘standard’ (usually inflated) price for some items, but also a ‘Clubcard price’ if you scan your Clubcard at the till.
Under 18s cannot have a Clubcard.
Therefore under 18s are being disadvantaged because of their age.
I’ve had a quick look at the discrimination act, and it looks to me like this is nailed on discrimination. Not sure how they can justify this!
YABU - Tesco are entitled to charge under 18s more for the same product
YANBU - This is unfair and discriminatory