Wahine, I've had both puppies and adult rescues and it's easier to deal with a dog that is past the chewie/potty stage of puppyhood. I think ideally a large breed that is 1 1/2 yrs to 2 years is still young enough, but able to be more focused and a little mellowed. Your mastiff won't be fully matured until 2 1/2 to 3 yrs of age.

Have you considered looking into a doggie daycare? One of my friends took her pooch to one that had a webcam set up and she could log in throughout the day to check on her dog. The daycare not only provided socialization, but exercise and snacks as well. Her dog came home happy and exhausted and couldn't wait to go back the next day!

As with any daycare, you'll want to take your time and research them. Check with your veterinarian or groomer who she would recommend. I would make sure the daycare put like sized dogs with similar temps in the room and that a staff member should constantly watch them and interact with them. Also, many daycares will want the dogs treated for kennel cough as well as being current on shots and altered.

Good luck and when you get that baby mastiff, we'll be eagerly awaiting puppy pics.