I am 5'4". I legitimately test as "large framed" via wrist measurement, etc. and I tend to carry extra weight in hips, thighs butt. I'd have be seriously skeletal to have a flat butt, it is just the way I am built.
I believe the lowest healthy weight on the BMI charts for 5'4" is 114. I could never ever weigh that.
My lowest adult weight was in the 120's when I was in my early to mid 20s.
At that age, I could easily maintain 135 without dieting. If I wanted to be restrictive or if I was extra active, my weight would go down to 125 - 130ish. Whenever I was in the 120s people would tell me I was too thin. Now...in no way were my ribs sticking out nor was I starving myself. I still wore a B cup bra, had breasts and of course a little butt
but I have photos of that time period and I was quite thin, especially my arms since I never did any strength training. I really was thin and would not ever have wanted to be skinnier than that.
After pregnancy and passing the age of 35, it seemed my "comfortable" weight was around 145 - 150. I was still thin. Not model thin, but thin. Nice curves and I like it that way. And I was in perfect health.
After age 40, I could diet down to about 150 - 155 but getting to the 140s even with regular strenuous workouts was just too difficult and I would have had to be way too restrictive to diet down that far. I'm happy with the way my body looked at 150 to 160 and if I can get back there now at age 50+ I'll be more than happy to maintain it.
SO...yes...based on my experience....under 120 seems very thin for someone my height....but if they are small framed, 115 to 120 could be a healthy weight for them.