Offically, in the sense that she was called that, I had a dog from birth. Quibble and I were born on the same day, and she was called my 'dog-sister' and refered to as my dog. Obviously, she did more to take care of me than I did to take care of her!
There were several of Dad's beagles that were called 'mine' as in I named them and played with them and tried to train them, but they weren't really mine either, and even I knew that. They were Dad's hounds, and moreover, I wasn't old enough for the responsibility.
The first dog that was supposed to be mine I was 12ish, and that was Mocky the dachshund. Honestly, I didn't do much to care for her. She was exceedingly hard to train (the breeder had told us she was selling a championship quality dachshund because she was too hyper for the show ring. No kidding. I'm not sure what possessed my mother to pick a dog for me that we KNEW would be difficult to handle, but she did. I think she had fond memories of the dachshund she had as a kid). I took care of her for a while, then refused to ever have anything to do with her again after she killed my cat. Mom took over. Dog never really was trained, but we lived on a farm, so it didn't matter too much.
Next dog was Sarama, and I'm 30. She truely is my dog. My fiance helps, but I have primary responsibility. I do feel that even if you live with your parents, if you are primarily responsible for the dog, if YOU are the one who usually walks it, trains it, feeds it, etc, then its your dog and not a family dog. Lots of people ask friends, neighbors, spouses, roomates, even children to help with the dog . . . it doesnt' make it not their dog.
As for getting a dog too young .. . I had lots of other animals from the time I left home. And it did put restrains on me . . . because of the cats, I couldn't be gone more than overnight, maybe two nights, without making arrangements. If I'd had Sarama, being as broke as I was, I would have been even more limited, since I wouldn't have been able to afford a kennel or daycare and would have had to depend on neighbors or friends to look after her if I had to go. That would have been a problem. But I think you have to make a choice. If you want to go out and party all night (And this is the only time in your life when you will otherwise be able to, so think hard) don't get a dog. If you want to have a drink with your colleges after work, don't get a dog (and you may miss a promotion because of it). You have to decide if you want a dog more than you want freedom. Since I'm sort of a homebody anyway (I'd really RATHER go home) and I have a fiance that lives with me for emergencies, I decdied that even as a first year associate at a law firm, I could handle it . . . I had, after all, waited twelve years for the income and housing that would allow a dog. And I'm really glad. But I wasn't giving up much . . . I'm inclined to come right home after work anyway. I'm not much of a party animal. So . . . take all that into account. If you want an animal, and you can't do a dog, get a cat. Or get a chinchilla, which was the creature I bought when I first started living on my own. She was great. She could be aloen overnight, even a litle longer. She was always glad to see me. She was soft and fuzzy and learned a couple of simple tricks.