Gemara

120a: When the Torah discusses Tzelafchad's daughters marrying, it lists them in order of their age. Here (when they asked for an inheritance), it lists them according to their Chachmah! This supports R. Ami:

(R. Ami): In matters of Torah, we give priority to the greatest Chacham. At a banquet, first priority is to the oldest.

123a: People talking on the crossroads said that Lavan's daughters will marry Rivkah's sons. The elder (Esav) will marry the elder (Leah), and the younger (Yakov) will marry the younger (Rachel).

Kidushin 51b (Mishnah): If a man was Mekadesh his daughter to Levi, without specifying which daughter, the Bogros (adult daughters) are not even Safek Mekudashos.

The case is, the Bogeres made her father a Shali'ach to be Mekadesh her, and said that he may keep the money. Even so, surely he was Mekadesh the minor, for that is a Mitzvah incumbent on him.

Rishonim

Rashbam (120a DH Lehalan): Presumably, Tzelafchad's daughters married in order of their age, for "we do not give the younger before the oldest!"

Tosfos (52a DH v'Hilchesa): R. Tam says that if one was Mekadesh 'your daughter' Stam, he was Mekadesh the oldest, for "here we do not give the younger before the oldest!" R. Menachem disagreed, for this is unlike Abaye and unlike Rava (neither holds that one is Mekudeshes to him). R. Tam retracted.

Rosh (Kidushin 2:25): R. Tam said that perhaps the Safek is only when he said 'one of your daughters', but if he said 'your daughter', he was Mekadesh the oldest. R. Tam did not say to rely on this in practice.

Poskim

Shulchan Aruch (YD 244:18): At a drinking fest or wedding feast, we follow seniority and seat the oldest at the head.

Bach (13 and Shach 13): A wedding feast is included in a drinking fest! Rather, this teaches that if brothers or sisters are having Nisu'in, we first make Nisu'in for the older, even if the younger is a bigger Chacham. We learn from Tzelafchad's daughters, who married in order of their age.

Maharit (Kidushin 51b DH umid'Amrinan): A Tosefta (4:12) says that if two brothers were Mekadesh two sisters, it is a Safek who was Mekadesh which, unless there were Shiduchin. We should assume that the older was Mekadesh the older, for if it not, neither couple could marry, for one of each couple has an older unmarried sibling! We can say that the Isur is only for a father marrying off daughters, but not for brothers who marry.

Note: One could say that the Isur applies even to brothers, and the older brother is already married!

Maharit (ibid.): When one was Mekadesh one of two daughters, both are Safek Mekudeshes. We must say that this is when their father was dead. Even though Tzelafchad's daughters were careful to marry in order after their father died, regarding Nisu'in surely it is honorable for the older to marry first, but we are not concerned about Kidushin (unless the father does it). We allow Kidushin during Chol ha'Mo'ed lest someone else be Mekadesh her first.

Note: The Mishnah says that there is a Safek even when the father was Mekadesh them!

Maharshdam (CM 318): Even though many argue with R. Tam, and he himself retracted, this is due to the severity of Ervah. Regarding inheritance (Reuven commanded that his nephew Levi will receive a certain sum if he will marry the daughter of his uncle Shimon), surely we follow the custom (he intended for Shimon's oldest daughter).

Maharsham (3:136): The 'Isur' does not apply to a son and daughter, for the son is commanded to have children.

Igros Chazon Ish (1:166): A case occurred in which a mother refused to marry off her younger son before the Bechor. This is improper.

Mishneh Halachos (9:242): We do not give a younger before the oldest, for the younger must honor the oldest. Lavan said 'we do not do so here', for the younger must honor the oldest. The father is not commanded. The Ramban (comments on Sefer ha'Mitzvos, Shoresh 2) says that honoring older siblings is a branch of Kivud Av v'Em, for the parents desire this. A younger brother need not honor an older brother's request 'do not marry before me', for both are commanded Pru u'Rvu. One does not honor a parent's request to be Mechalel Shabbos! It seems that an orphan girl need not wait for her older sister to marry (since she is not commanded to honor parents). However, Tzelafchad's daughters married in order of their age even after their father died! This is because their mother was alive. (They asked 'if we are not like sons, our mother should do Yibum!') Therefore, each was commanded to honor her older sisters. (The Rosh expounds the Mitzvah to honor an older brother from "v'Es Imo".) This suggests that the Halachah applies not only to the father. Rather, even orphans should marry in age order. Tzelafchad's daughters all married above 40 years of age. It seems that the Halachah applies to only to daughters, who are not commanded Pru u'Rvu, but not to sons. However, the Shach connotes that it applies even to sons.

Igros Moshe (EH 2:1): Surely, a man may marry before his older brother found a wife. The younger is commanded to marry. He may not neglect this because his brother did not yet fulfill it! Even in a case of Ones, i.e. the older did not yet find a proper girl, we cannot tell the younger to wait for an unknown period of time, especially if the father (of the girl who wants to marry the younger) does not want to wait. Some say that the Shach forbids. This is a total mistake. He discusses when both brothers made Shiduchin and are about to marry, and teaches that we honor the older and marry him off first, even if the younger is a Chacham. This Halachah is brought in Yoreh De'ah in laws of Kavod Chachamim. Tzelafchad's daughters all found Shiduchin at the same time, therefore, they married in order of their age. Kidushin 51b proves this. If David was Mekadesh his daughter to Levi Stam, the Bogros are not even Safek Mekudashos. Even if the Bogeres made David a Shali'ach to be Mekadesh her, and said that he may keep the money, surely he was Mekadesh the minor, for that is a Mitzvah incumbent on him. If there were any Isur or even if were slightly unethical, he should be Mekadesh the older one first! He is commanded to marry off a Bogeres just like a minor - "Benoseichem Tenu l'Anashim" applies to both. If a man died, we assume that he wanted each daughter, even a Bogeres, to receive a 10th of his estate to marry her off, due to this Mitzvah.

Rebuttal (Mishneh Halachos 9:242): The Rashbam says that Tzelafchad's daughters married in order due to 'we do not give a younger...' We learn from Lavan, and Leah did not have a Shiduch. Yakov desired Rachel, and worked for her! We are concerned that brothers marry in order only when they are not commanded Pru u'Rvu, e.g. one marries them off just before adulthood, or the younger already fulfilled Pru u'Rvu through a previous wife. All the more so a man need not wait for his older sister, for she is not commanded Pru u'Rvu.

Igros Moshe (ibid.): The Gemara means that the Mitzvah to marry off a minor is only on the father, but a Bogeres is also commanded to marry herself off. We say that surely he was Mekadesh the minor, even though the Isur to marry off the younger first was said regarding daughters. All the more so one need not wait for his brother to marry, for he has no obligation to marry off his brother! Even though R. Tam says that even without Shiduchin, we assume that he married off the older due to "here we do not do give...", surely this is not an Isur. It is merely normal to be Mekadesh the younger when both are minors or both are adults who made him a Shali'ach. Even if the older is pained to see his younger brother marry first, he should not be pained. This is the Isur of envy! The father may help the younger brother, unless he suspects that this wil make the older sick. We cannot forbid the younger to marry. If David says 'if you will not give to me a gift, I will become sick', one need not give to him (unless he is obligated due to Tzedakah). The same applies here. We need not be concerned lest he embarrass his older brother; his brother brings this upon himself. Also, there is no need to be embarrassed, for many delay to marry for many reasons: they want to learn before they are burdened with a household, or they want to establish an income first. Others may assume that the older delays for such a reason. One should marry a girl below his own standing (Yevamos 63a). If he is haughty and waits for a prestigious girl who desires him, his younger brother need not wait for this. It is unreasonable to forbid someone to engage in his own bodily needs, lest someone else be embarrassed that he did not succeed like him. Certainly one should not postpone the great Mitzvah of Pru u'Rvu, lest he be punished for not marrying by the age of 20.

Mishneh Halachos (9:242): Perhaps there is no Chiyuv for a father to marry off a youger son, if an older son is single, due to "here we do not give..." Perhaps it is improper! The Bach and Shach teach that the father should not marry off the younger first; the younger child may do like he wants.

Yabi'a Omer (4 EH 6): R. Tam did not rely on his reasoning in practice. Regarding sons, who are commanded Pru u'Rvu, even he would agree that the younger may marry before the older. However, if a man was Mekadesh on behalf of one of his minor sons, who are not yet commanded, this does not apply so much.