Although most parents and professionals agree that fluoride is good for the prevention and reduction of tooth decay, fluoride can be toxic when swallowed. Toothpaste with fluoride is not recommended for children under the age of two, (non-spitters - infants and children up to 3)
Most toothpastes with fluoride are not recommended until then at least and sometimes later (it's usually written on the back of the toothpaste tube). I guess it's just harder to control the amount babies are swallowing.
hope this helps!