Great question and a complicated one. Hard to have that type of discussion on a board such as this- it's an emotional topic. What you have described above is very ambigous.
Biologically/medically speaking, puberty is a good place to start. To keep it simple, 12 is a good marker for boys. If you are/been the parent of a 12 year old boy, you are familiar with the physicaly transformation that occurs from one season to the next. Boys go through puberty later than girls. One year the girls are bigger, the next that advantage is completely erased. It's why, generally speaking, girls teams will scrimmage against boys teams 2 years younger. Boys quickly close the gap when they hit HS. Clubs that use younger boys as a training tool for older girls usually transition to 3 years younger when girls are U17.
The eye test is easy to administer. Again, emotionally charged...we as a society are trending towards the emotionally charged discussions VS a scientific and pragmatic ones. An athletic, high leve U15 boy has no business on the same competitive field as an athletic, high level U15 girl - it's not even close. We can carry this over into hormone treatment.....but..... what are we doing having discussions about hormone treatments for teens who's brains aren't 100% developed and can barely make decisions on what shoes to wear for school <--------This is type of statement is where the societal and cultural clash occurs...
You will never ( I should probably not say never) be able to apply NCAA, IOC, etc guidelines to youth sports...and should you? Are parents OK treating pre-pubescent children and to what end?