ive been suffering from disordered eating my whole life and actually never experienced amenorrhea from it! my body fat is around 20% despite being at a 15 bmi, the only reason i dont get them anymore is because im on hormone replacement therapy taking testosterone and even that took about 2 years to work for my bleeding. everybody is super different and what our bodies can handle absolutely varies, i know some people who lose their period when they get to a 20 bmi, but they also lack a lot of fat on their body which is a trend i notice in people who report experiencing that vs people who dont. i remember reading stories of lean women who exercised a lot, yet never dipped into being underweight, and still lost their period so thats why i suspect its more of a body fat thing rather than just being underweight. (btw iirc means if i recall correctly!)