Brains fully haven't developed until 16 yrs of age and Favre says kids shouldn't play tackle football until at least 16 yrs old. Here's the story: https://www.washingtonpost.com/news/early-lead/wp/2018/04/12/brett-favre-says-he-suffered-probably-thousands-of-concussions/?utm_term=.7679c5200be1