At least 6 months old befor water
Breast milk and/or formula provides all the hydration a baby needs until they're 6 months old. Even on hot days do not give a baby under 6 months any water and do not water down formula. You could give them and extra bottle or nurse more frequently if you're worried about dehydration. Because babies live on a liquid diet at this age the water can fill their tummies which will ruin their appetite and deprive them of the nutrients they would get from milk. It also messes with their sodium levels which can cause serious damage to their system.
Even after 6 months old water should be given sparingly. My doctor said no more then 2-4oz in a 24 hour period. Basically just sips while learning to use a cup or to wash down any solid foods if necessary.