My 5 month old is in the 100th percentile & wears 18 month-2T clothes. Fairly big for his age, sitting unassisted, & doctors recommended we feed him solids. Just to give context for size, he sits in the comfort seat comfortably & loves his time in the wagon. My 5 yr old does not use the comfort seat & does not care for it. But I love the harness to give extra support & safety for my 5 month old.