Then let that training guide you. I have a kid nearly six and just short of 40 pounds. I follow essentially the same rules as Bestfriend on the old 33 foot racer, because it snaps around a lot. We allow no PFD below on the new-to-us full keeler because the motion is significantly different, and in waves of any size he's in his bunk watching a DVD anyway. On deck or on dock, it's ALWAYS with a PFD, because his swimming is still a little lacking, and being a skinny kid, he's got a buoyancy deficiency, so to speak.
If he's on deck in heavy weather, he gets clipped on. Being a kid, he tends to get himself lower (right down to lying on the deck) the harsher it gets. He seems immune to seasickness.
His only hesitation is in stepping off the boat (about an 18 inch drop) because occasionally we are blown off a few feet, as we are at a finger end and I prefer a bit of space. But he already knows to stand on the breast line to bring the boat closer to the dock, but at 40 lbs. and a 13 tonne boat, that takes a little while to work! ("DADDY! MAKE THE BOAT MOVE IN!")