I took my daughter and her BFF a couple years back when they were 13. There were older kids there and younger so you won't be the only one with a 7 year old.
It was LOUD!! Not just the music, but a lot of these girls scream the entire time. The girl behind us was absolutely piercing! That was my girls' only complaint. Other than that they had a great time.
The rule in our house is that you must be a teen to see a live concert. A rite of passage. :) When they were younger we took them to plays, musicals, ballet, ice shows, etc.,but behavior at live concerts is different than other events. Often even young teens are dropped off without parents and behavior can get a little out of hand. At the JB concert we went to the biggest issue was excessive rude/pushy-ness (even in a venue with assigned seats) and an overload of swearing. No issue with drugs and such.
Ultimately it is your call. Every family must make these decisions for themselves. I love that you came here to get opinions! This site is a great way to get a wide variety of information and opinions, so we can make these kinds of decisions with our eyes wide open. :)