There are a number of points to consider here.
Firstly, while the increase in the risk of developing dementia looks very large (about 365%), the actual risk is pretty low to begin with (about 0.65% if I recall correctly). So, there is no need for any kind of panicked reaction.
More importantly, the study shows that footballers actually have an increased life expectancy over the general population, so overall, being a footballer is a huge benefit.
The study is also, of course, looking at players who played in the '60s and '70s mainly, so you have to take into account the change in tactics then to now. The ball spends much less time in the air.
As for banning it in training for u12s, that seems pretty sensible to me. My lad, who is 12, plays at a high level (Championship team academy), and they have never had more than a couple of training sessions on heading anyway. The actual number of times they head the ball during games is already pretty small. Basically, they do it when they need to. They attempt at all times to play on the deck and training is predominantly about ball skills, so not allowing them to head balls in training isn’t likely to have that much of an impact.
Overall, I think it’s a good idea to protect the developing brain from too much unnecessary trauma, but no one should panic about this issue.