A dog crate can be a great tool for training your dog, but there are some basic rules for Dog Crate Safety. How Young is Too Young?
Young puppies can be introduced to the crate, but puppies less than four months old should not be kept in a crate for long periods of time without supervision. Of course, if he crawls in there to take a nap, that's one thing, but young puppies should not be locked in a crate during waking hours.
One reason for this is that the puppy's body isn't mature enough yet to hold it. Much like children, dogs must reach a level of maturity and readiness before potty training occurs. The other reason is that puppies can't get all of the attention and playtime they need if they are confined to a small space without interaction.
How Long is Too Long?
Puppies can gradually be left alone for short periods of time. The older they get, the longer they can safely be left in the crate alone. However, this doesn't mean you can leave the dog for 12 hours a day and expect him to be healthy and naturally train himself. Start with short periods of time, like 20 minutes. Work up to an hour and eventually several hours at a time. It is recommended that dogs not be left in their crates for more than 6 consecutive hours. 
A good rule of thumb for puppies is that they can keep potty muscles in control about an hour for each month of their age (meaning a two-month-old
puppy should be taken out every two hours, and so on).
If you must work long hours, have a neighbor or family member check on the dog once or twice throughout the day to let him outside, give him time to run around a bit, and get a drink of water. If there isn't anyone you can count on to help you out with the puppy, consider using a dog-sitting service that will come to your home, or enroll him in a doggie daycare.
When is it Dangerous to Leave the Dog in the Crate?
There are a few instances when crating is not a good idea. First, the dog should not be crated if he has outgrown his crate and it is too small for him. Second, if the dog is sick or if it is very hot. In these cases, the dog should have free access to fresh water and air. Also, when considering dog crate safety, remember a dog shouldn't be made to sit in a mess if he gets sick while you are away.
