The Talmud in מסכת ברכות has a section where various great Rabbis give advice to their children as they enter adulthood and set out on their paths in life.
One example of such advice was given by the great רבי יהושע בן לוי. He tells his children to be careful with the honor they must give to an elder who was once very learned, and forgot his Torah that he once knew.
Rashi explains that his forgetfulness could have come because of illness, or the stress of years of struggle to earn a living.
Whatever the case may be, such an elder is compared to the שברי לוחות, the broken pieces of the first set of Ten Commandments. These were collected and placed in the Holy Ark with the second tablets and they were not discarded. רבי יהושע בן לוי told his children that they must never discard this תלמיד חכם. They must remember to show him the greatest respect.