astute - Ability to understand things quickly and clearly, and to use the knowledge well
From a very early age, Gail instinctively knew everything about business, and and after college she proved to be an astute business woman, and became a millionaire before the age of 30.
senile - Used to describe behavior, mental states, or conditions associated with aging, especially when a person becomes forgetful or confused.

The elderly man showed symptoms of becoming senile, like forgetting where he put his keys, but with support, he managed to live independently for a while.