Can teenagers aged 19 years old get dementia?

Yes. One point in case is mad cow disease acquired recently in europe and other places by young people. Otherwise is rare. Probably other dementias or diseases like kuru kuru affect young people too. In youngsters we think first in delirium and not in dementia. Delirium is more acute or sudden, caused by drugs, medicines, toxins, alcohol etc.
Extremely unusual. Nearly everyone with dementia is older and a key part of the diagnosis is loss of abilities that one used to have. 19 is pretty young to diagnose someone with that. A traumatic brain injury could lead to a dementia at any age.