Well, there has not been any study dedicated to determining what age to start giving Yakult to dogs. However, Yakult is good for the health of fogs in general, therefore, I would advise seeing a vet before giving Yakult to your dog regardless of what age.
Although in some reports, it was stated that small size dogs should be given about a teaspoon of Yakult per day, 2 teaspoons of Yakult to a medium-sized dog per day, and 3 teaspoons of Yakult per day to a large-sized dog.