Most, if not all, the dementia cases we read about are players who played using the old 'heavy' ball. The modern ball is a lot lighter and more importantly, is waterproof and doesn't get even heavier when wet. I think banning heading during training is reasonable but banning it in matches is a no no.
I played until I was 40, all with the old ball. Centre-half, always heading the ball but I don't remember ever heading the ball in training (unless there was a practise match, which wasn't very often.)
The line on the graph will start going down when players who played all their careers with the modern ball are taken into account.