Most of the studies that showed rapid mental decline in old age have been debunked. A 70 year old with no actual problems (e.g., Alzheimer's) is very likely to function perfectly fine mentally.
A 2002 study (published in the Alzheimer's and Dementia Journal) found that ~91% of people 70 and older were capable of normal mental function.
A 2002 study (published in the Alzheimer's and Dementia Journal) found that ~91% of people 70 and older were capable of normal mental function.