Select the most appropriate one-word substitution for the given group of words.

A person who is experiencing dementia brought about by old age

Senile

The most appropriate one-word substitution for the given group of words "a person who is experiencing dementia brought about by old age" is senile.

• Senile means showing the physical and mental decline associated with old age.
• Sentinel means a person or thing that stands guard or keeps watch.
• Primal means relating to the earliest or earliest known stages of development.
• Primitive means relating to the earliest stages of human development or culture.

The word senile is the most appropriate because it specifically refers to a person who is experiencing dementia, which is a decline in cognitive function that is associated with old age. The other words do not have the same connotation of being a person who is experiencing mental decline due to old age.