In reply to marsbar:
> Its more about the shape of the child than the age. If s/he has a defined waist and hips so that if they were upside down in a waist harness it would stay put then thats fine. If s/he is straight up and down and a waist harness would not stay put if the child inverts then a full body harness is needed.
Its also about head size. As children grow, their heads (very dense parts of the body) become proportionately less significant which lowers their centre of gravity which reduces the probability of an inverted fall.