Our kids were 12 and 9 for the paddle with this company, and they each were in a double kayak with one of us. I don't recall if there was an official minimum age stated - though I expect children a bit younger than ours could handle the trip when paired with an adult. For kayaking in general, I don't think there's any one best answer - depends on the personality/physique of the children and the type of kayaking you plan to do. For a several-hour fairly leisurely paddle, our kids started at 8 years old when in a double kayak with one of us, or 12 if on their own in a single kayak (though we were nearby).
Get quick answers from Kayak Annapolis LLC staff and past visitors.