The by-laws of your Association will state the permitted ages of people allowed in the pool unsupervised.
Vandalism is not allowed under any rules that I know of.
The kid's friends are guests of his parents, effectively, so they are responsible for their actions.
The “I am an owners so I can do what I like” argument is simply not applicable in any strata. “I am an owner and I have to obey the rules or face the possibility of fines” would be more correct.
The association should be issuing Notices To Comply for each of the breaches of by-laws.
The police might intervene in cases of vandalism and threats or personal abuse but otherwise they would rightly say that there is a mechansim for dealing with this within the strata so it really isn't any of their business.
Date-stamped photographs of the boy and his mates misbehaving might be in order for future action.
Being called a troublemaker is a bit of a badge of honour in strata – hey, that's what people call me.