The Torah introduces the Mitzvah of "Mipenei Seva Takum Ve'hadarta Penei Zaken," which is a law that requires us to; stand in the presence of, and give respect to, Torah scholars and elders. This means that when a Torah scholar or elder comes within four Amot (6-8 feet) of a fellow, the fellow must stand and remain standing until the scholar or elder leaves his four-Amot radius.
The question arises, what constitutes an "elder" for purposes of this Halacha? At what age do we consider one an "elder" insofar as the obligation to stand in his presence and afford him honor is concerned?
Maran (author of Shulchan Aruch) and most Poskim (authorities of Halacha) are of the opinion that a person becomes an "elder" at age seventy, while the Ben Ish Chai (Rabbi Yosef Chaim of Baghdad, 1835-1909) cites the Arizal (Egypt-Tzefat, 1534-1572) as claiming that the obligation applies already from the age of sixty. Strictly speaking, Halacha follows the view of Maran and most other Poskim, that the Mitzvah applies only to those aged seventy and above. But, Halacha says of one who wishes to be stringent and follow the Arizal's view, "Tavo Alav Beracha" he is deserving of blessing.
Very often, one sees an older person but cannot ascertain whether or not he is older than sixty or seventy. What should one do in such a case when he cannot determine the age of an older person?
Chacham Ovadia Yoseph applies to this situation the principle of "Safek De'orayta Le'chumra," meaning, when an uncertainty arises concerning a Torah obligation or prohibition, one must act stringently. In this situation, when the Torah obligation of "Mipenei Seva Takum" is at stake, one must stand, even though he is unsure whether the older person has reached the age at which this obligation applies.
Therefore, for example, if a person sits on a bus or train and an older person walks within four Amot of him, he must stand. It should go without saying that if the older person has no other seat, one must stand and offer his seat. Though this might seem obvious, unfortunately, in today's culture of selfishness, many people lack this basic ethical principle, and do not offer their seat to an older person on a bus or train.
By the same token, we must train our children to show respect to not only those over the age of sixty or seventy, but to adults in general. Every so often there is overcrowding in the synagogue or at community functions, and there are more people than seats. Parents should train their children to stand and allow an older person to have their seat. But when it comes to people above the age of seventy, or, according to the Arizal, above sixty, there is an actual Torah obligation for even grownups to stand and offer the older person his seat, and whoever does so first is credited with a Mitzvah.
In summary, one must stand in the presence of Torah scholars and elderly people aged seventy and above when they come within a four-Amot. It is commendable to be stringent and stand in the presence of adults aged sixty and above. If one is unsure whether an older person has reached age seventy, he must assume that he has, and stand in his presence. This obligation also requires offering an older person his seat on buses and trains, in the synagogue and at community functions.