I'm fifty. I'm not the same guy I was when I was forty, or thirty, never mind twenty, or ten. I visualize identity not as a solid object but as a wave form travelling along the temporal dimension through a complex emulsion of memories, experiences, and emotions, bounded at front and back by singularities—boundaries beyond which there is no continuity (and almost certainly no persistence of identity). We're all waves travelling through this common soup of human existential phenomena, occasionally refracting through one another and being changed thereby. And as we move, we change. Not only are our physical bodies not made up from the same individual atoms: the bits you could notionally use to describe us change, too. New data is added, old patterns are lost (I have the memory of a goldfish these days).
Beyond the obvious (gross physiological deterioration and pathologies of senescence), what are the psychological symptoms of ageing?