Unless you buy a monocle, adopt a pseudo German accent and supplant your middle name with "von", I don't think using "Dr" at 25 will go down very well. I would suggest you put it on your e-mail footer, so that it is understated yet obvious - but you'll come across as a pretentious pratt if you insist on being called Dr. Besides that, most firms have a first-name culture - so unless you want people to call you Dr Dave, I'd stick with Mr.