6 months. Younger babies normally do not drink water. They just drink breast milk or formula when they get thirsty. Water is a thinner liquid than milk, so babies sometimes have trouble drinking water (but they drink milk just fine). With older babies that are already on solid foods, they can try a bit of water sometimes during mealtimes.
5 months. Generally, a baby gets plenty of water from breast milk or fornula, with the proper electroyte balance(very important) giving a small amount is ok but never let a baby drink large amounts as this could lead to what's called water intoxication where the baby's sodium level drops too low and may experience seizures. Your md may recommend pediolyte to replace water lost from vomiting or diarrhea.
4 months. A good time to start is at 4-6 mos, the same time you start complementary foods (usually cereal.) just offer a little tap water from a sippy cup (a bottle isn't necessary.) this heps establish a (hopefully lifelong) routine of drinking water during meals.