An "elder" is not an "office" per se but rather a family and societal role in Semitic societies (which continued into Pauline assemblies). That is, elders were the alpha males of families who represented the families in the broader congregation:
NIV Numbers 11:16 The LORD said to Moses: "Bring me seventy of Israel's elders who are known to you [IE: recognized by you] as leaders and officials among the people. Have them come to the tent of meeting, that they may stand there with you.
This was not unique to Israel:
ESV Genesis 50:7 So Joseph went up to bury his father. With him went
up all the servants of Pharaoh, the elders of his household, and all
the elders of the land of Egypt,
NIV Numbers 22:7 The elders of Moab and Midian left, taking with
them the fee for divination. When they came to Balaam, they told him
what Balak had said.
The 70 elders of Israel were to be the ones recognized by the people as leaders and officials and were trusted by the people to represent them at the "tent of meeting".
The alpha male of the family would always be a male and always of significant age, hence the name "elder" which in the Greek was literally an "older man" and age commanded respect:
NIV 1 Timothy 5: 1Do not rebuke an older man harshly, but exhort him
as if he were your father. Treat younger men as brothers, 2older women
as mothers, and younger women as sisters, with absolute purity.
However while the elder of a family would be a senior member (and never what the ancients referred to as a "whipper-snapper") or a woman there could be an exception in the case of a family bereft of a competent representative because of a lack of marbles in the most senior's marble cache and such. 
Elders serving in the wider congregation must not be a man with more than one wife since he might favor one family unit over the other and the less loved wife might go unrepresented and would probably be overburdened and thus unable to be devoted to a more public role:
ESV 1 Timothy 3:2 Therefore an overseer must be above reproach, the
husband of one wife, sober-minded, self-controlled, respectable,
hospitable, able to teach,
NIV 1 Corinthians 7: 32I would like you to be free from concern. An
unmarried man is concerned about the Lord’s affairs—how he can please
the Lord. 33But a married man is concerned about the affairs of this
world—how he can please his wife— 34and his interests are divided. An
unmarried woman or virgin is concerned about the Lord’s affairs: Her
aim is to be devoted to the Lord in both body and spirit. But a
married woman is concerned about the affairs of this world—how she can
please her husband. 35I am saying this for your own good, not to
restrict you, but that you may live in a right way in undivided
devotion to the Lord.
So it is not possible to vote a person into such a role. It is a question of recognizing the voice of those who are the legitimate family leaders. You can't make a young person or a woman an elder by democracy. The respected elders of the community are recognized and charged with public leadership and the others defer. They received public endorsement and responsibility. That is the way it was from ancient times. If you visit tribal societies you will find the same today.