Inflammation, insulin resistance, toxic load, nutritional deficiencies, hormone imbalances are just a few of the things which alter production of important trophic factors such as BDNF (Brain-derived Neurotrophic Factor). When too many variables fall below optimal levels, the brain shifts from plasticity to neuronal pruning.
Dementia doesn’t appear overnight. The underlying pathology is a gradual process which is taking place for many years (decades) before issues become apparent.