Depends on intent. You can begin to introduce a baby to a playful pool experience as early as a few weeks, as long as you pay strict attention to the babies breathing and comfort. This can help avoid fear of the water in the older child. The drawback is you must always closely monitor a childs poolside behavior as they age. Limited understanding of pool dangers mayresult in tragedy. You cannot drownproof a child.
As early as you want. You can start bringing your baby to the pool as early as you want. 6 months and up seems to be a reasonable age. Plenty of shade is a must and do not have the baby outdoors for hours in extreme heat. Hydrate well and watch for good wet diapers. Keep in mind that most recommed limiting pool time early in life due to potential links with chlorine and asthma (especially indoor pools).