Young onset dementia is often referred to as early onset dementia.

A person developing dementia before age 65 is said to have young or early onset dementia.
Young onset dementia is a broad term which describes symptoms such as loss of memory, intellect, rationality, social skills and normal emotional reactions.

It is very rare for people to show signs of dementia before the age of 40, however, it is possible.
Plenty of cases are reported where children have been diagnosed with dementia.
In August 2018, a 2 year old boy was reported to have Sanfilippo Syndrome, a rare genetic condition that causes fatal brain damage and leads to childhood dementia. The life expectancy for affected children is between 10-20 years old.
MPS III – Sanfilippo
If you are interested, you can do some more research on this. The Society for Mucopolysaccharide Diseases provides an explanation via their website. Click on the blue button if you would like more information.
