I played midget football growing up and it started about 5th grade as I recall. I don't remember a lot of kids getting their bell rung as much as I recall some broken bones and soft tissue injuries, and there really weren't many of those. Just my opinion, but I think the kids were a little tougher back in those days ;)! There was a weight limit of 120 lbs, so the really big kids had to wait until Jr. High school to play if they couldn't cut the weight. In that way, I think it helped reduce the number of injuries involving hard contact. We were lucky to have some coaches that emphasized the fundamentals and being good sports was a requirement. The city recreation director was great and made sure to keep any of the over-zealous or down right crazy ones from coaching. I go back to being taught proper fundamentals, especially with tackling, as a reason there weren't a lot of injuries in that lower league. Have your eyes up and face mask on the football, contact with the shoulder, etc.
Some of what I see in the little leagues these days can be dangerous and it looks to be a result of having coaches whose mentality is to win at all costs. Living vicariously through the youngsters robs them of the opportunity to grow up learning the right (and safe way) to play the game and can thrust them too soon in a hyper-competitive situation which they most likely aren't ready for. If it hurts them physically and there is little enjoyment or personal success, they likely lose interest never to play again. I agree with
@stephenPE that middle school (7th grade) is early enough for contact. But if you want to introduce it sooner, make sure your child has a coach that can teach him properly and doesn't overmatch him.
There's always going to be a risk of injury to the head and other body parts when playing football or any other sports involving bodily contact. I don't have a problem with those who don't wish to participate as a result. I do, however, have an issue with those who would seek to keep those from participating who want to.