I took my DD to an orchestral concert, in a well known venue. The venue advertised it as for 7+, and she is 8 years old. It was an evening performance.
Granted, when I got there, there were virtually no children, and the average age of the audience was over 60. I didn't see it as an issue though, as the venue had a clear age policy which included her.
She sat nicely through the performance, not a peep. The couple on one side made passive aggressive comments, including complaining that she ate snacks during the intermission (something about it 'lowering the tone'). She was only allowed to eat during the intermission, not the performance. The snacks were purchased from the venue (I see it as important to support them in a time when the arts are struggling financially)
I accept that people have paid a lot of money for their tickets, and therefore want to enjoy the performance, but if a child is within the venues age policy, and not disruptive, surely this is fine?