Of course association is not causality. Just several studies sending signals. Not dissimilar to smoking, you won't be seeing high-level evidence in the form of a prospective randomized trial as it would be unethical. In any case, this much is clear: Loud music puts your future hearing in peril. Yes, we need more studies to make the dementia connection more clear, but I have enough LOW-LEVEL evidence/signals to say be careful with noise, if only to keep your hearing in the future.