Upon performing or buying alcohol?
Anybody can perform in a pub. The restrictions are upon buying alcohol.
That said, a pub can deny entry to any person they like. Their invitation to enter is one that can be withdraw in their sole discretion unless it offends against the Equal Opportunities legislation.
There are child labour laws but they wouldn't be breached here.
Can I clarify anything for you?
Not on performances.
I suppose it is possible but not likely.
All the best.