The Challenge 25 policy is total overkill even for alcohol, let alone bloody paracetamol.
I've been able to legally buy alcohol for almost 20 years and the rules, which were already pretty strict when I was in 6th form (ID to get into any pub or club or buy alcohol in a shop, only passport and driving licence accepted) have got progressively stricter.
I was once ID'ed in Sainsbury's at the age of 28 for buying a bar of Bailey's flavoured chocolate, which would have had less alcohol in it than orange juice, but that isn't an age restricted product (yet).
Other western countries, with the exception of the US, don't do this.
And do they have serious underage drinking problems which are magically absent in the UK and the US?
No. Quite the reverse, in fact.
The whole policy, and the UK's entire attitude towards alcohol, needs a complete rethink. Or maybe the state treats British people like children when it comes to alcohol because they behave like children when it comes to alcohol. No real need to apply that to painkillers and cutlery though.