I don't think this has to do with liquor laws per se. The minimum drinking age is 21 in the US. I'm assuming it a policy of not having non adults (under the age of 18) unaccompanied in the lounge. To the OP a letter probably isn't going to do it. If I were you I'd dress in a polo and fairly nice jeans, simply walk in, smile, saying good.... and present the card for entrance and hope for the best. At age 16 some young teenagers look more like age 21. Since we don't know what you look like I'd suggest giving the appearance of a young adult.