In centuries past, children started, learning combat skills from the time they could walk, but in the context of a modern sport, there is such a thing as too young.
The children’s class we run is a mixed age, mixed ability group. There is a recommendation from the FIE (fencing governing body) that age 8 is a suitable age to join in group classes, both for safety’s sake and for beginning productive lessons. That’s the minimum age we’ve tended to start kids at Sway and in other classes we have run in the area.