An adult.
In a legal context, peer most often refers to anyone who is an adult citizen, such as in “a jury of one’s peers.” Generally, a jury of one’s peers is a random selection of other citizens from a similar geographic location.
These individuals do not necessarily share traits similar to a defendant’s. As such, there is no guarantee that those who hear a defendant’s case will be of a similar age, race, socio-economic background, or gender as the defendant, though these traits alone cannot disqualify someone from serving as a juror.