More examples

She has five children, the youngest of whom is still an infant.

He was very unwell as an infant, and was not expected to live into adulthood.

Separated from his mother when he was just an infant, the boy grew up in a children's home.

She isn't able to look after herself at all - she's as helpless as an infant.

I'll give you this booklet to read, which gives advice about feeding infants of under four months.