The Torah (Vayikra 19) states: “You shall rise before the elderly and honor the face of an old man and you shall fear your G-d, I am Hashem.” Our Sages (Kiddushin 32b) explained that “elderly” refers to an individual who is old in age and “an old man” refers to a Torah scholar; even if he is young in age, one must rise before him completely to one’s full height in order to show him respect. This a positive Torah commandment.
We have already explained that rising before one’s primary rabbi or the leader of the generation refers to rising to one’s full height as long as one sees one’s rabbi walking until he sits down or until he disappears from one’s eyesight similar to the law regarding a Sefer Torah where the entire congregation in the synagogue and in the women’s section must rise completely while it is being carried from the Aron Kodesh (Ark) to the Bimah (center podium) and from the Bimah back to the Aron Kodesh (as Maran Ha’Shulchan Aruch rules in Chapter 282). Nevertheless, regarding a regular Torah scholar who is not one’s primary rabbi and is not the leader of the generation (although one has studied Torah from this individual), one must only rise before him when he enters one’s four Amot radius (approximately 6.5 feet); when the Torah scholar leaves one’s four Amot, one may be seated.
Maran Rabbeinu Ovadia Yosef zt”l states that those individuals who do not rise completely before a Torah scholar and make due by rising slightly are acting incorrectly, for the Rambam and Maran Ha’Shulchan Aruch rule that one must rise completely to one’s full height before a Torah scholar. Most Poskim agree. Thus, one must indeed rise completely before a Torah scholar.
Nevertheless, if a Torah scholar does not wish for others to rise before him, one need not do so, for as we have already explained, a Torah scholar has the right to absolve others of his honor.
Maran zt”l adds that although one may be seated after a Torah scholar has left one’s four Amot, nevertheless, if a Torah scholar boards a bus or train and does not have a place to sit, one must rise before him and vacate one’s seat so that the Torah scholar can be seated. The same applies to an elderly man. One may not stay seated and allow the Torah scholar or the elderly man to remain standing although they are not necessarily within one’s four Amot.
We should point out that many times when a group of young Kollel men would enter Maran zt”l’s Bet Midrash to hear a Torah class from him and Maran would notice an elderly gentleman among them, he would request for this man to be seated in a respectful place and sometimes even right next to him out of respect for the individual’s advanced age. Similarly, when respected Torah scholars entered Maran zt”l’s Bet Midrash, Maran would take care to rise slightly before them out of respect for their Torah knowledge although they had not reached anywhere near Maran’s level of Torah knowledge and fear of Heaven.