Sounds like me: puberty at 15-16 (but I didn't think of it as delayed), similar height, slight skeletal structure, neotenous face with weak chin/jaw and sad/emotional eyes, weak/small muscles, "hardgainer", brain fog, sleep schedule.
I blame it on poor genetics on my mother's side, constant abuse from parents, and malnutrition. I also think my dad tried to prevent me from getting too strong and fit to keep me docile and obedient; he would always discourage sports, weight training, martial arts.
I trained religiously for a few years (diet, lifestyle, training, research, even quitting alcohol, sugar, smoking, and fapping/porn) and gained a decent level of strength and increased my bodyweight from 63 kg to 75 kg. But I still don't look good, look like an easy target, and the average male could probably make the same progress in a few months.
I think this physical condition was huge factor in becoming FA. I felt timid and retreated into books and videogames, and started getting very behind my classmates in social experience and missed so many milestones.