Although King Lear is an Elizabethan play, its setting is not. It was supposed to represent much earlier times. However, Shakespeare still brought Elizabethan issues into the play, and the "current events" of the times into the plot.
One of these current events pertained to the treatment of the elderly by society. In Elizabethan England, the elderly, the higher classes, the aristocracy, and the ranked were to be unavoidably respected. Yet, Elizabethan England was falling apart after the death of QE I. Therefore, as James I came into the show and began doing his own thing, the values of Elizabeth's society began to fall apart and were evident in several legal cases that Shakespeare witnessed as he saw the transition from one kingdom to another.Most of them included mistreatment of the elderly-something unheard of by then.
You might want to check Shakespeare's biography to see how he tends to do this very thing in every one of his plays.