That sucks man. Explain to them that M is 17+ so the game is meant for you. They can't 'shelter' you forever. Personally I don't care what the rating is, since I can buy it either way..lol..but I just hope they make it good!
DD was T. BD was M. I'm guessing M for DD2. If they really won't let you buy it come here and we'll help brainstorm ideas for you to get it! (of course once your 17 you can just buy it and don't need to tell them...)
and welcome to the forums!