Under-18s can still enter licensed premises. At least here in the UK they can. And they often do, if my local that is more "5-year-old's playground" than "pub" is representative.
That's usually not the case in the US. It depends on the jurisdiction, but bars can oftentimes be fined for underage drinking if a minor is on the premises, even if he/she isn't actually drinking.
Even in situations in which the venues have discretion (eg, nightclubs, comedy clubs), venues often don't allow underage patrons to enter, because they know they're going to make no money off of them, and they're just taking up space that could instead be going to a paying customer.
Interesting, I wasn't aware that this was permitted in the UK.
Here in the US, under-18's often aren't allowed on the premises of a drinking establishment at all, so the OP does raise a point depending on the location of the conference.
