My Vet had told me that their bladder is not fully formed until 16 weeks old....
I think what she meant was they don't have bladder control until they're at
least four months old...
I found this online:
Bladder control and your puppy
To put it simply, your little guy just can't hold it — at least not for very long. A puppy under the age of 4 months doesn't have a big enough bladder or sufficient muscle control to go more than a couple of hours without eliminating. As he gets older, though, a pup's ability to control himself gradually increases. And by the time he reaches adulthood (at about one year of age), a healthy dog usually has plenty of self-control.
Cluing into a Puppy's Housetraining Instincts - For Dummies