Another question for you, Cannimelism, is are you going to be able to afford the cost of the dog's feeding and care? Have you done the math on what a mastiff will cost you annually? Food, treats, vaccines, flea/tick/heartworm preventative... it does add up quickly, even more so for a mastiff just because of their size and appetites. And you also need to think about how you would pay for any urgently needed vet care (illness, injury). Those bills can rack up into the thousands of dollars very quickly.
Not trying to be a downer, but people not thinking about the costs associated with getting a dog (especially a large breed dog) is why a good number of dogs get dumped into shelters.
Obviously I don't know your financial status, but you mentioned you are not even eighteen, and most eighteen year olds that I know in their first apartment can barely feed themselves and do not have thousands of extra dollars per year to take care of a mastiff.
For example my Nadia, a 100lb DdB, eats a $22 bag of food per week (she eats 4Health at Tractor Supply- not the cheapest, not the most expensive) That's $1144 a year. Add in the basic care (rabies, distemper, bordetella, hw test) at a pet store vaccine clinic, that's $120. Now we're up to $1264 a year. Add in flea/tick preventative at about $11 month for another $132. Now we're up to $1396. Heartguard for a year is about $95 (more if your dog is over 100lbs and you need two packages). Now we're up to $1491 a year. That doesn't include anything fancy, or treats, or a license, or detailed veterinary care. You're looking at about $125 a month for just the bare essentials.
Just food for thought...