מפני שיבה תקום והדרת פני זקן (קדושים יט:לב)

The Ksav Sofer explains this Pasuk allegorically, based on the Gemara that points out that there are two separate commandments in this verse: The first one is “You shall rise in front of the aged,” even if this person is not a Talmid Chacham.  The second Mitzvah, “you shall show respect in the face of an old person,” deals with someone who has acquired wisdom; the word זקן is turned into two words: “זה”-“קנה” This one has acquired – referencing to a Talmid Chacham, even if he is a youngster.

Therefore writes the Ksav Sofer, in the first part of this Pasuk it says מפני – in front, meaning as soon as you see the elderly person approaching, do show due respect.  Whereas when a young person approaches, you only need to stand up when you discern from the way he speaks that he is a Talmid Chacham.