What is the difference between "Seivah" and "Zakein"?
Ramban #1 (citing Toras Kohanim and Targum Onkelos): "Zakein" is synonymous with 'Chacham', 4 an indication that one only needs to honor and rise before an elderly Talmid-Chacham.
Ramban #2 (citing Kidushin, 32b) and Targum Yonasan: One is obligated to rise and honor an old man - even if he is a Bur, and a Chacham, even if he is a child. 5
In general terms, a person who has turned sixty is a Zakein, and he becomes a Zakein at the age of seventy (See Pirkei Avos, 5:21).
See Sifsei Chachamim.
Ramban: See also Sifsei Chachamim.
Ramban: The acronym of Chacham is 'Zeh she'Kanah Chochmahan).' (See Ramban). See also Beha'aloschsa, Bamidbar 11:16.
The Ramban suggests that maybe this is also the opinion of Targum Onkelos - only he incorporates a young Talmid-Chacham in the word "Seivah", whereas "Zakein comes to include an elderly person irrespective of his knowledge.
See Kidushin, 32.
What does honoring a Zakein entail?
Rashi: It entails not sitting in his place, not speaking on his behalf and not contradicting his words.
Why does the Pasuk add "Veyareisa me'Elokecha"?