Dementia is described as the deterioration in cognitive function. This is different from the normal declination of cognitive health which is the consequence of biological ageing. While dementia is mostly seen in older people, it is not an old age related issue.

from Times of India https://timesofindia.indiatimes.com/life-style/health-fitness/de-stress/signs-of-dementia-know-them-for-timely-medical-intervention/photostory/91465579.cms