Necessary if tough viewing
The things that make I Am Not Your Negro tough to watch (in particular images of violence inflicted on Black people throughout the periods the film covers) are things that are necessary for people to know about. I think that the film is suitable for teenagers to watch by themselves (and have watched part of the film with my 11 year old niece), but that you get the most out of it watching in a group setting (in class or with your family) for discussion purposes.
One of the events the documentary covers is the racist bombing of a church that killed four little girls. I think a child the same age as those poor girls should be able to watch this documentary because of how it shows that systemic, VIOLENT, anti-Black racism didn't care about protecting innocent children - because to them, Black children weren't innocent.
I've worked in education and I have experience working with teens and young adults on similar material and I think that if you're not sure if your teen can watch this documentary on their own: watch it with them. Do your homework so you can answer their questions and then sit with them so you can learn together.
There ARE images of lynching that are upsetting to see in maybe the last third of the film and that may upset smaller and/or sensitive children, but I do think that hiding the reality of what people have been/are like (as lynchings have occurred as recently as last year) is doing a disservice to your kids and to the history that Black people in the US have survived.