I went last year for the first time when I was 16. I was only allowed to go for the whole weeked because my dad was there (he goes to all the festivals, he didn't camp with me and my firends though) but if he wasn't there I would have only been allowed to get day tickets and not camp.
I think it depends on how mature your daughter is and if you think she can be trusted to behave. Some of my friends weren't allowed to stay for the weekend as their parents believed they were too young, however most of them were allowed to. If a reason you don't want her to go is because it's camping/staying there alone with friends maybe allow her to get day a ticket? (so she could go on the day and come home on the night) thats what my friends mams and dads let them do if they weren't allowed to camp. :-) x