First of all, you are not underage. You are merely too young at the moment for a concealed permit, which I'm guessing is why (among many other possible reasons) is why you are legally OCing instead of illegally CCing. Now if the cop specifically asks you if you are old enough to be OCing, answer him in one word or less. If he says he does not believe you it might be necessary to show him some proof of your age instead of being offered an all-expenses paid trip to the station to let them run your name and prints. You decide how you want to deal with the issue.
Positive attitude (you know you are doing nothing wrong so have no reason to act as if you did anything wrong) goes a long way. Be polite but be adamant about your rights being respected. (If the cop wants to violate your rihts the street is not the place to fight it out. A recording of the interchange will be good for your side when you follow through with your official complaint and possible lawsuit.)
While others may disagree, I do not like asking if I am being detained - it may give a cop an idea he had not already formed. I'll decide just how obvious it is that I am being detained and proceed from there. If I'm in cuffs, or up against he wall with my feet spread I'm probably being detained. If the cop is standing in an "interview stance" and asking me questions I'm probably not being detained. If I'm not being detained I'll just start walking away as if I had a specific place to be other than there with the cop. If he wants to change my mind he will let me know.
Cop asks me about my handgun in my holster. I mention how warm/cold/dark/bright it is. He wants to know why I'm carrying and I tell him what I had for supper. Cop asks me what's up with my smart-ass answers and I tell him I have no obligation to talk to him and obviously am choosing not to. That is all done with a smile and a pleasant voice. (Ys, I have been in training about that lately.)
But that's just me. YMMV.