Let's try an analogy. The UK age of consent to sex is 16. While under-16s do have sex, we consider that they aren't able to make a competent, informed decision about doing so before that age, and we - as a society - are particularly concerned where that happens with an age gap. In other words, it would be illegal for any under 16 to have sex but more likely for action to be taken if it's with a 30 year old than with, say, another 15 year old.
Given all that, do we feel that under 16s are competent to decide they want to take their clothes off in a large mixed group of people? Or would we say that might be a decision where they might feel pressured and coerced? And how would the fact that the group will contain adults much older than them, rather than being a group of their peers, affect that too?