Found these research articles stating that fruits and veggies combat cognitive decline and age-related depression. Good reasons to go vegetarian or at least a higher proportion of vegetarian food in one's diet in middle or old age. As Hippocrates stated, one's food and diet is the best medicine against disease. More Fruits and Veggies May Lower Depression Symptoms in Aging Adults Frontiers | Increased Consumption of Fruit and Vegetables Is Related to a Reduced Risk of Cognitive Impairment and Dementia: Meta-Analysis Associations of vegetable and fruit intake with cognitive function and its decline: Two longitudinal studies - ScienceDirect
I think one big reason why eating more vegetables is healthier is due to the bad foods they replace, as well as the actual nutrients they contain. This factor is often overlooked in most nutrition studies. Bad food feeds bad bacteria which cause everything from mood swings to upset bellies to Crohn's disease to cancer and probably many more illnesses they haven't found the cause in the gut yet. Those bacteria control more than our digestion and physical wellbeing, but our mental state as well. It's all about your gut and the bacteria therein.