The lightest I've been in my full-grown life is 55kg/121lbs and I'm only 160cm/5'3". Heck, I'm the same clothes size now as I was then (NZ/UK 10, which I think is a US 6) but I'm 5kg/11lbs heavier. It's not like I'm seriously muscled or anything, I just must have been made of something different when I was 18 than I am now.