It really does depend on the person.
I am 50kgs (height is about 162cm)and for me that is a healthy weight - i have hovered around this size all my adult life. I am thin but i am not a stick figure, i am not starving myself - this is my natural, healthy weight. My body doesn't gain weight (unless i'm pregnant) i always remain around the 50-52kg mark.
I know on others this weight would be ridiculously underweight though - it is just way too variable.