I imagine there may also be issues with insurance/duty of care.
If an adult gets injured and it's their own fault e.g. climb on stage then fall off, they have responsibility over their own actions and the consequences. Now imagine if a 15 year old got injured. Mummy and daddy would then sue the heck out of the venue.
Also the standing concerts can get quite jam packed. If a young teenage falls down, they could genuinely be trampled.