If I understand you correctly, you are suggesting that the age of dementia onset is getting lower. That is actually not the case. The Framingham Heart Study found an increase in age at dementia onset (1.5 years/epoch) and a decrease in years lived with dementia (1 year/epoch) over time. One of the reasons is a rising education level and better treatment of cardiovascular risk factors. As you can see from the risk factor illustration in the post, these two trends seem to be reflected in the increasing age at dementia onset. For cancer, though, you are right in pointing out that for certain cancers there seems to be a declining age of first incidence. The suspected cause is a worsening diet.