I think it’s just more common now, Ellanvannin, because people are living longer. The chances of getting it do increase with age.
It’s hardly new, though. I remember reading an Arnold Bennet novel, one of the Clayhanger series, where he describes in some detail the growing signs of dementia in Clayhanger’s father. Bennet must have had first hand experience, IMO - he describes such things as forgetting how to use a knife and fork, and inappropriate but utterly uncharacteristic behaviour with a housemaid.
That was set around the late 1800s/very early 20thC IIRC - in those days it was called ‘softening of the brain’.