It absolutely is. Section 146(4) of the Licensing Act 2003 affords a defence for selling alcohol to a child if the person making the sale reasonably thought they were 18 or over. In contrast, in most States in the US selling alcohol to a minor is a strict liability offence.
I think our system is fairer, especially to shop workers on minimum wage who shouldn’t be penalised and criminalised when they have taken all reasonable precautions.
I would say the same about people who have sex with minors, given the sentences involved. I don’t see how that’s fair if someone reasonably and genuinely believed they were having consensual and legal sex that they should be punished on the basis they they knew.
What is reasonable, as ever, is up to a jury, who have listened to the evidence in order to determine whether they are sure of someone’s guilt; which is the least worst way of deciding these things, rather than via a football forum.