What is the minimum age
For what exactly???
Kids under 12 get in for free, but they have to be accompanied by an adult (over 18) at ALL times. If any child is found on their own at ANY point by staff/security they will be classed and treated as a missing person! Which could a) cause problems that cause you to miss bands or b) get their responsible adult to take them off site potentially meaning you miss the remainder of the festival.
From 13 upwards they have to buy a ticket, so they may well be questioned about not being 13, which is only a matter of 1 year! I would suggest that you take copies of their birth certificates to save any of you being disappointed, cos if they dont get in then you have to deal with them before you can go in without them!
If you are 15 or under, you need to buy a ticket AND be accompanied by an adult (over 18) at ALL times.