We taught our son to snorkel around 4.5-5 years old.
A parent gave me valuable advice - keep it simple. Let them start with just a mask first, add the snorkel once they are comfortable and save the fins for later. A well fitting vest that doesn't ride up around their face is essential. I think we got his vest at BassPro, but that was 15+ years ago.
Have fun, I miss the younger years. Snorkeling is something we still enjoy together and our son is a beginner diver now. I am considering giving it a try so we can do it together on vacation.
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