The earliest age to start snorkeling with a child will vary based on their comfort in the water and how well they can follow directions.
There are certain mechanics they need to learn to effectively breathe through a tube, but more importantly they need to stay composed and trust that they can breathe and float.
This generally consists of a kids snorkel mask and a children's snorkel. We bring our own kids snorkeling set when we travel, as many tour companies or rental outfits just don’t have the right sizes for young children.
Our kids always wear a lifejacket when snorkeling – we wouldn’t consider otherwise for them and consider it an essential component of child snorkel gear.