Historians refer to them as the contemporaries/a contemporary of the x century, or the x century contemporaries/an x century contemporary.
contemporary: a person living around the same time.
How many obscure, unintelligible points, which we judge too easily as incredible, were natural and clear as daylight for the contemporaries of those past ages!
Scholars have cited so many reasons - simple and complex - that we have sometimes forgotten what the contemporaries of those times observed.
Lawyers of the 17th century doubted it and the contemporaries of the 15th century were very careful indeed in defining its legal mature.
Which of the following aspects of the Dutch society most impressed 17th century contemporaries?
However, only a few of 19th century contemporaries were sounding the alarm about the particular female undergarment.
Riis was a contemporary of the rapid urbanization of the
late nineteenth century, during which time cities became expanding centers of industry.
Alternately, one can also refer to contemporaries of a given century as people contemporaneous to..., or our ancestors/forebears of...
forebears, also spelled forbears: ancestors, forefathers.