Interesting. BJJ for fun (not competition - so start in position and do less full speed take downs) is probably one of the least impact inducing sports I've ever done. Of course getting bumped in the head can happen, but it's usually with newer people. I mostly stick to higher belts and people I know, and I can't think of the last time I got hit in the head.
Running and basketball I had to quit because of impact on my body overall, though not so much my head. Wakeboarding I quit b/c of a torn ACL and minor concussions in my 20s. Skateboarding is obvious lol. Surfing doesn't have very much impact, but in big waves there is a whole other set of risks.
Anyway, I was just a bit surprised by your comment b/c out of all the sports I've done, BJJ seemed like the one I could do for a very long time just by sticking with people I know and trust.