My son is 30, and still staying with us. Recently he has been spending a lot more unnecessary money (eg when he doesn't like our cooking, he will order food). Should I start charging him rent instead?

Yes you should, else he will take u for granted

I have encoutered a similar scenario, except that the troublemaker is my brother - not helping for houseworks, play games till mid night and shouting occassionally, etc.

I send him few warnings and eventually asked him to move out - it is not a pleasant experience and I was blamed by my parents for doing so, however it turns out to be a good decision.

My relationship with my brother becomes much better now after he moved out - probably he has more freedom outside and doesn't need to bear my nag anymore...