"Seivah" is never less than 70 years.
Kidushin 32b (Beraisa) Suggestion: Perhaps "rise in front of a Seivah" includes even an Ashmai (ignoramus) elder!
Rejection: The verse also says "Zaken", which connotes a Chacham - "gather for me 70 men from the Zekenim of Yisrael."
R. Yosi ha'Gelili says, "ZaKeN" denotes one 'she'KaNah' (who acquired) Chachmah - "Hash-m Konani Reishis Darko."
R. Shimon ben Elazar says, "Zaken and you will fear" teaches that a Zaken should not unnecessarily cause people to rise for him.
Isi ben Yehudah says, "rise in front of Seivah" refers to any elder.
Question: The first Tana and R. Yosi ha'Gelili agree!
Answer: They argue about whether or not one must rise for a young Chacham. R. Yosi holds that the Torah interrupted between "Seivah" and "Zaken", to teach that one can be "Seivah" without being "Zaken", or vice-versa. The first Tana holds that the Torah refers to one person (an old Chacham).
(R. Yochanan): The Halachah follows Isi.
R. Yochanan would rise for Nochri elders, because they have experienced many afflictions and many miracles. Rava would not rise for them, but he would show honor. Abaye would give them a hand. Rava and Rav Nachman would send messengers to give them a hand.
Rav Nachman thought that it would belittle the honor of Torah if he himself would help them.
Rif and Rosh (Kidushin 14a and 1:53): The Halachah follows Isi.
Ran (DH Omar): The Halachah follows Isi. We rise in front of even an Ashmai elder, i.e. an ignoramus, but not a Rasha, [and all the more so an idolater. R. Yochanan] would stand in front of old Nochrim.
Hagahos ha'Bach: The bracketed words should say 'like it says that R. Yochanan.'
Ran (ibid.): R. Yochanan did so for the sake of Shalom. Rava showed honor, and Abaye gave a hand, but they did not stand for Nochrim. Rashi connotes that Isi holds like R. Yosi, that Zaken refers even to a young Chacham, for one must stand for an elder even if he is not a Chacham. However, it seems that the Rif rules like the first Tana, who exempts standing for a young Chacham. The Torah wrote Zaken to put it next to "you will fear" (to teach that a Zaken should not cause people to rise for him). This is not clear. Throughout Shas we say that we stand in front of a Chacham. According to the Rif, it depends only on age! The Ramban defends the RIf, and says that the Gemara always refers to standing for old Chachamim. The difference between an old ignoramus and an old Chacham is that a Chacham need not stand for the former, for it is below his honor. This is not clear. Shmuel stood for Rav Yechezkeil, for he had great deeds (33b). I say that we must stand for Chachamim only because Chachmah generally brings to great deeds! The Torah did not command to stand for such people, for usually it is not evident. One must stand for a young Chacham is only if he is greater than himself, but not if he is smaller, and perhaps not even if he is equal. Chachamim of Bavel stood for each other. Rashi explains that they learned each other's teachings, so each was like the other's Rebbi, for whom one must stand from when he sees him.
Rambam (Hilchos Talmud Torah 6:9): If one is extremely old, we stand in front of him, even if he is not a Chacham. Even a young Chacham stands in front of an extremely old person. One need not stand totally, rather, just enough to honor him. We honor even an old Nochri verbally, or extend a hand to support him. "Rise in front of Seivah" refers to any elder.
Yere'im (36, cited in Mekoros in Frankel Rambam): We stand for a Nochri only if he keeps his seven Mitzvos.
Rosh: R. Tam explains that 'Ashmai' refers to an unlearned elder. Rashi says that it is a wicked, ignorant elder. This is wrong. Isi would not obligate honoring a Rasha. One may hit and curse him, for he does not act like your nation! What do the first Tana and R. Yosi ha'Gelili learn from Seivah? If not for Seivah, one might have thought that Zaken refers to any old person. Since it says Seivah first, we learn that Zaken must be a Chacham. Alternatively, both of them require rising in front of an elder who is not ignorant, even if he is not a great Chacham. Since Isi expounds Seivah to refer to even an ignorant Chacham, he must expound Zaken to refer to a great Chacham even if he is young. Someone above 70 is called Seivah.
Shulchan Aruch (YD 244:1): It is a Mitzvah to rise in front of any Chacham, even if he is young, even if the Chacham is not his Rebbi.
Beis Yosef (DH Kasuv): Shibolei ha'Leket says that the argument is about a young Chacham below Bar Mitzvah. Tosfos and Semak say that one stands only for a young Chacham outstanding in Chachmah.
Rema: One stands only for a Chacham greater than himself, from whom it is appropriate to learn.
Shach (2): The Ramah says that one must stand only in front of one greater than Stam people. If not, an ignoramus would be obligated to stand in front of everyone, and this is totally unreasonable.
Shulchan Aruch (ibid): Also, it is a Mitzvah to rise in front of a Seivah, i.e. who is at least 70.
Beis Yosef (DH u'Mah she'Chosav v'Chen): We learn from Pirkei Avos (5:27), that Seivah is 70.
Birkei Yosef (4): The Ari Zal says that based on Kabalah, there is a Mitzvah to stand for someone above 60.
Halichos Olam (8 Ki Tetzei 5): Hadrei Da'as says that the Ari Zal holds like the Rif, and obligates standing for a Chacham above 60, and even an ignoramus above 70. Birkat Yosef says that the Ari Zal obligates standing for one above 60 who is not an ignoramus, even if he is not a Chacham, and for anyone above 70. Really, the Ari Zal holds like the Zohar, which says that 60 is Seivah, unlike our Mishnah in Avos. We rule like our Gemara (Bava Basra), which says that Seivah is 70; it is good to be stringent like the Ari Zal.
Rema: This is even if he is ignoramus, as long as he is not a Rasha.
Shulchan Aruch (7): Even a young Chacham stands in front of an extremely old person. He does not stand totally, rather, just enough to honor him. We honor even an old Nochri verbally, or extend a hand to support him.
Beis Yosef (DH v'Afilu): The Tur holds that R. Yochanan did not totally stand for old Nochrim; he rules like him. It seems that also the Rambam explains this way. If so, we must say that Rava (and Abaye) did not stand for them at all. Even though the verse says 'stand', it does not equate an old Ashmai Nochri to a Chacham Yisrael. Tosfos says that the verse does not include Nochrim. This is why R. Yochanan needed to give a reason (they experienced many afflictions...) He did not say that he rises because the Halachah follows Isi! The Rambam rules like Abaye and Rava.
Lechem Mishneh (Hilchos Talmud Torah 6:9): R. Yochanan was a Chacham. It was below his honor to stand for elders. Therefore, he explained why he stood. Why didn't the Rambam rule like Rava against Abaye? Because there is such an extreme argument between R. Yochanan (who himself stood) and Rava (who did nothing himself), the Rambam rules like Abaye, who is an intermediate opinion (he extended a hand).
Bach (DH v'Afilu): The Tur holds that R. Yochanan stood. Abaye and Rava hold that we should not equate Nochrim to Yisre'elim. Rava showed honor, i.e. he rose only a little. Abaye gave a hand. The Tur rules like them. The Rambam holds that Rava verbally honored them, and rules that one does like Rava or Abaye.
Taz (4): The Tur says that a young Chacham does not disgrace himself to totally stand for him. Other people totally stand for him, like for a Chacham.
Eshel Avraham: Beis Yehudah (YD 28) says that all the more so, one stands for an old woman.
Shulchan Aruch (8): Two Chachamim or elders need not rise for each other, but they show honor.