This is now more than a decade old, but can highly recommend Frontline's "A League of Denial: the NFL's Concussion Crises" (free to stream as a great documentary on the beginning of that research regarding CTE. More details have come out since but it gives you a really solid grounding on the problem, done by investigative reporters making sure they get the science right.
While it is overwhelmingly focused on the NFL, it does also get into how dangerous football is for young kids. Which they could research because they tragically have some kids die young for unrelated reasons and researchers got to look at these teens and 20 somethings brains. And they had signs of damage. Even in some people that had never had a concussion reported.
Basically its cumulative damage via deceleration... not just concussion. Kids that haven't gone through puberty yet are particularly susceptible to damage done *below* the threshold for a concussion because they haven't fully developed neck strength. So anything with a lot of sudden sharp deceleration via collision where they may effectively get whiplash (or just below diagnosable whiplash) is the culprit.
So it's not the time they got a concussion, it's doing repeated tackling drills with the sudden stop. The helmet doesn't help, because damage is due to brain moving inside skull due to the sudden stop. Helmet helps vs collision vs a secondary object but not just sudden stops.
Teens and adults are at lower risk because of greater muscle mass that can absorb more of the force safely, but still AT risk because when they DO tackle... they have a lot more mass which means a lot more force is involved. Its sheer physics. A little more resistant to the low speed collisions, but engaging in far more high speed, high force ones.
Which is also some of why football is more dangerous than boxing. You've got full body mass accelerating/decelerating.
Force = Mass X acceleration
a boxer isn't using his full mass or running speed, so even though its a direct head strike... its still got less force applied than quarterback getting plowed by a linebacker. And just by the nature of boxing, routine practice is not involving head strikes or rapid deceleration of the head with near the force of someone plowing into a tackling dummy.
Overall young kids shouldn't play tackle football. It's not necessarily a hit in game that's the risk, its repeated drills in practice that aren't done in a safe way. With insufficient rest in between. brains are really delicate and heal slowly from even minor trauma you can't see.
teens & adults likely aren't getting near enough rest to recover either, especially for routine practice cumulative stress. There's techniques to make it safer but it's never really safe because of the cumulative nature of damage without time to fully heal in between.
Football is, unfortunately, often the only way some teens can get scholarships to try and get out of where they are... all while destroying the ability to benefit from same scholarship. Same kind of trap as military recruiting.
Switch sports if you can. don't get into the football pipeline. That goes as viewer too. Find something else. don't feed the beast. there's probably a delightful minor league team for something nearby that you can learn to take joy in. Take younger folks with you to a non-football game and let them learn to love something else as well.