Its almost certainly the aging process. My mom is 93 and experiencing the same.
I had modest success with my mom's vascular dementia (in her eighties, passed at 91) through vit. methyl-B12 (given as 3 crushed 1000mcg sub-lingual tablets/day, under her tongue). It's a mild nerve restorative. When first started I noticed she recalled things she'd been forgetting for a year or two prior -- I think the net effect aside from intitially reversing her decline by a few years, was to greatly slow her further mental decline.