When my oldest boy was 4, I'd take him on 3 to 4 mile hikes and expect to carry him on my shoulders for the last mile or half-mile. He is 5 now and after a growth spurt, has graduated to a waist-pack that can carry his snacks, a quart of water, and whatever gear he wants to bring (usually a head-lamp for caving and a walkie-talkie and whistle for if he gets lost). In another year or two, I'll let him carry a small pack on overnights.