The minimum age limit with PADI to get a certification card that allows you to go diving in open water (instead of a pool) is age 10 although there are some limitations on depth etc. that are in place until the age of 15 where a full Open Water Diver certification can be obtained.
If you simply wanted to try it out in the pool most PADI Dive Shops offer courses for 8-10yr olds (PADI Seal Team or PADI Bubblemaker)
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