This kind of objection often comes up, but I think is based upon a misunderstanding of what it means to be a "peer" in the required (legal) sense. One is my "peer" if one is a fellow citizen with all associated rights and responsibilities. That person doesn't have to be my equal in strength, or intelligence, or at basketball--he or she simply has to be my equal as a citizen. If their vote counts as much as mine, they're my peer.
In his Republic, Plato said (with evident contempt) that democracy was something like government by "bald-headed tinkers." (I resemble that!) But at the heart of democratic theory is the idea that all people are "created equal," by which the theorist cannot sensibly mean "equal in all things." The point is that we are all, or at least should be all, regarded as politically and legally equal. Other political theories--including especially Plato's--obviously reject this idea. Plato especially thought that political decisions--just like all medical decisions--should be made only by those with appropriate expertise. It is a little difficult to identify just how we would identify this expertise, however, how we could produce it in a class of rulers (or judges), and how we could avoid corruption of such a system.