He is known as an expert in gerontology.

An old person was walking in the park.

An old person was playing with his grandchild in the park.

He couldn't say anything about issues outside of his field of expertise.

There are many different specialty stores in this town.

My grandmother lives in a nursing home.

He is a specialist fool, with no interest in anything outside his field.

My grandfather is interested in gerontology.

My grandmother participates in the local old people's club every week.

He lives in my brother's family's house.