Yes I agree with that. I didn't hit 100 pounds until highschool and yet I got my first period at 12.
Not sure about the 105 pounds thing....according to the growth chart the pediatrician gives us every year, my daughter will likely be 105 pounds and 5'1" at most if she follows the path she's been on forever, when she is 18!
Also not sure about the body fat thing. They send a paper home every year from school (PE) called a fitness gram and it measures their body fat. Her % is always in the 14-16% range. So I don't know if she will ever get to the over 20%.
She is 12 1/2 and 4'9"...probably weighs about 80 pounds now. She has started developing...probably about a year now....she has a few friends who got their periods and likely weighed about 80 pounds.....so, I think anything is possible.
I think it is also body type....my daughter has a very petite bone structure....tiny wrists and fingers....but she will, of course, get her period even with a small bone structure!
It's one of the few things out there that is still a true unknown..and that can be challenging for some parents...that not knowing!