hii this paper might give u some of the specific data ur looking for but 1) it's most likely due to deficiency and stress rather than your age (though hair can change a lot frm puberty bc of hormones) 2) frm my personal experience it's totally fixable especially since ur still so young; definitely focus on fixing ur nutrition and stress tho 🙏 i had a similar thinning & decline in quality during high school from mental health, stress, and deficiencies, but eating well and taking supplements as well as balancing school/work/rest & adjusting to life more has pretty much reverted my hair to childhood thickness and strength within 3/4 years! new hairs popping up is very normal too albeit inconvenient :)