Can a person have dimensia and then not have it 7 years later?

It depends of the type of dementia. There is also a condition of the brain that lay people refer to as "water on the brain". It exhibits the same symptoms as normal dementia. An MRI can determine if a person has this condition rather than classic dementia. 

Dementia is forgetfulness. This may happen due to sudden injury to brain. The neurons may stop working in the area of memory. The brain slowly recovers and memory can come back. In the case that part brain gets rotten and then it will never come back. I think that this region is in the middle of the top of head which can easily undergo memory changes. Yes. He can get it back even after 20 years if brain cells did not die (neurons

No, because dementia (note the spelling) indicates brain atrophy, and since brain tissue doesn't regenerate, it's permanent.