הלכה על קידושין 64:17

The commandment to honor sages: To honor sages and to rise in front of them, as it is stated (Leviticus 19:32), "Before an elder rise" - and Onkelos translated [it as], "Before one who understands the Torah rise" - "and dignify the face of the aged (zaken)." They, may their memory be blessed, explained (Kiddushin 32b), "A zaken is only one who has acquired (shekanah) wisdom. And the reason that the verse expressed, the sage, with the language of "aged," is because the young sage has seen with his wisdom that which the aged has seen from his many years.