Sentence:  It is hardly necessary to remind students of covered bridges that Timothy Palmer was born in 1751 in nearby Rowley ; that he moved with his parents to West Boxford when he <WSD>was</WSD> sixteen years old ; and was there apprenticed to a builder and architect , Moody Spofford . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)