Differences between senile dementia and altzheimers disease?

They are the same. It used to be thought that alzheimers only happened to young people-what we now call early onset alzheimers dementia. It was also thought that getting senile happened to all older adults. We now know senility and dementia are not normal aging.
Category/type. "senile dementia" is the everyday expression for a class of disorders (dementias) that occur in the elderly population. Alzheimer's is a common type of dementia that typically occurs in elderly people.