There's tons of factors more than just nutrition. Stress is a big one. My 9yo niece just got hers, and they've got plenty of money and eat well. Came shortly after they found out her dad is riddled with cancer all over his body.
i remember years ago an article said it was from eating pork (genetically quite similar to human) which had tons of hormones injected so they would grow up faster for more profit turnover. i wonder if theres puberty age data from groups that dont eat pork.