Start at the first page of DisBoards, then scroll down past the Disney boards and you will come to one Sea World/Discovery Cove. Will answer all your questions.
Been twice and loved it there. We did not do dolphin encounter and did have discounted tickets.
Yes there is an age minimum for dolphin, not sure on snorkel.....but the water is cold, deep and I would suggest a wet suit and vest for any child, even if they swim well. Then they (and you) can focus on the fish not swimming.