I can only speak about one top 20 school and experience ranging over 20 years.
Members are medical school faculty members. This can include both physicians and non-physicians. It can include scientists and non-scientists (e.g. faculty from the medical humanities such as ethics, law, literature). Some of the members may be in their 30s and some may be emeritus faculty in their 70s. The committee may include medical students who have a role in conducting interviews or reviewing applications after interviewers.
The adcom members I know have included men and women, gay and straight, young and old, a variety of medical specialties, from many different religious traditions and politicial opinions.