Gemara

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.

(Rav Ashi): This is only if he is extremely Chacham or old.

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 "here we do not give the younger before the oldest!"

Tosfos (52a DH v'Hilchesa): R. Tam says that if Ploni said that he would give his daughter Plonis to David, and David was Mekadesh 'your daughter' Stam, surely he intended for her, for "Bnei Yisrael will not do unjustly." If they never discussed a particular daughter and he was Mekadesh Stam, we say that 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 says that if Ploni made Shiduchin that his daughter Plonis will marry David (and David was Mekadesh 'your daughter' Stam), surely she is Mekudeshes, for "Bnei Yisrael will not do unjustly." One does not pass over a Mitzvah to do an Aveirah, especially if Plonis was there at the time. (When there were no Shiduchin,) 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.

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.

Ohr ha'Chayim (Bereishis 29:26): How could Lavan claim "here we do not give the younger before the oldest"? Yakov stipulated that he works to marry the younger! Perhaps Lavan said this only to justify himself to the people of his city. Or, he claimed 'I never agreed to violate the custom. I agreed to give Rachel after Leah married. Since you asked 'give to me my wife', I thought that you meant Leah!' Yakov held that the custom is not binding; Lavan was required to reveal his intent. In truth, all Lavan's claims were refutable.

Poskim

Shulchan Aruch (YD 244:18): At a drinking fest or wedding feast, we follow seniority and seat the oldest at the head. If the younger is an outstanding Chacham, and the elder is not so old, the Chacham comes first.

Bach (13 and Shach 13): Why was it necessarily to specify a wedding feast? This 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.

Maharsham (3:136): A case occurred in which Shimon had daughters from his wife Leah, and after she died he married Esther and had a daughter (Sarah) from her. Shimon died, Esther remarried, and they found a Shiduch for Sarah. Leah's oldest daughter (Chanah) says 'I must marry first.' The questioner said that since the father is dead, he does not give the younger first, and there is no Isur on the daughters. The Rashbam disagrees, for he says that Tzelafchad's daughters married in order of their age, even though their father was dead. For such matters, we learn from what preceded Matan Torah. However, this case is pressed circumstances, for Sarah is already 22, and Chanah is 27 (and we do not know when she will marry)! Surely, the Isur does not apply here, for Sarah found a Shiduch with a rich Chacham. Who knows if another such Shiduch will arise?! The 'Isur' (for the younger to marry first) is mere Derech Eretz. Even though it is brought in YD 244, it is not an absolute law. We learn from Tosfos that one may rely more on "Bnei Yisrael will not do unjustly" than on the Isur to marry off the younger daughter first. The Taz (YD 169:19) says that in pressed circumstances, one need not fulfill "Bnei Yisrael will not do unjustly." If so, the same applies to marrying off the younger one first.

Shulchan Aruch (EH 37:16): If one was Mekadesh his daughter to Levi, without specifying which daughter, the Bogeres is not even Safek Mekudeshes. This is even if she made her father a Shali'ach to be Mekadesh her, and said that he may keep the money.

Aruch ha'Shulchan 46) Why aren't the Bogros in the Safek when they made their father a Shali'ach? Surely it is a Mitzvah for their father to be Mekadesh them. It is normal to marry off the oldest first. Lavan said "here we do not give the younger before the oldest." How can we disagree? R. Tam wanted to say so, but Chachamim of his generation disagreed, and he himself did not do so in practice. It is not clear why.

Chasam Sofer (Likutei Teshuvos 5): It is difficult to transgress what is explicit in the Torah. Rashi (Bamidbar 36:11) explains that Tzelafchad's daughters married in order of their age, to uphold the custom from before Matan Torah. My Rebbi Noson Adler said that a case occurred and he stumbled.

Tzitz Eliezer (22:52): The Shulchan Aruch and Rambam do not explicitly forbid to give the younger before the older. There is only a custom. Rishonim and Acharonim gave weight to this custom. Maharsham's Perush on the Torah (Techeiles Mordechai) connotes that there is a real Isur; he explains like the Chida that Yakov wanted to marry Rachel first, for she and Leah were twins, and he held that she is really the oldest (she was coinceived first - see Rashi Bereishis 25:26). However, Teshuvas Maharsham (3:136) says that it is only etiquette and proper conduct. He also says that it does not apply to a brother and sister, for only men are commanded Pru u'Rvu. Ginzei Yosef (120) says that all permit when the older sister is not the Bechor. He permitted a case in practice, in which the older sister consented. Beis Yechezkeil (60) says that we are careful not to marry off the younger first only if the father gives a dowry to his daughters. Bi'ur ha'Rif on Teshuvas Mahari Mintz (60:12) says that Mahari Chagiz disapproved of this custom. We should learn from Yakov, who wanted to marry the younger first, and not from Lavan! Ha'Rif concludes that if the father can marry off either first, he should marry off the older. Mahari Chagiz discusses one who found a Shiduch only for the younger.

Chelkas Yakov (EH 41): R. Tam assumes that one was Mekadesh the oldest, due to "here we do not give...". Why do we assume that a man was not Mekadesh his Bogeres? One does not do an Isur if he can accomplish his goal in a permitted way! We must say that even those who forbid marrying off the younger first hold that it is a mere custom with a support from the Torah. Chachamim concluded that due to the Mitzvah to marry off a minor, he intended for this (it overrides the custom). We say that one does not do an Isur (if he can accomplish his goal in a permitted way) only regarding an absolute Isur. Surely, if one was Meshadech Leah, and later was Mekadesh Stam, he did not intend for the older (to fulfill "here we do not give...") at the expense of retracting from Shiduchin. This is when he said 'your daughter.' If he said 'one of your daughters', even if there was Shiduchin, perhaps he was Mekadesh another.

Mishneh Halachos (9:242): In Kidushin, we say that surely he was not Mekadesh a Bogeres? Why don't we apply 'here we do not give...'! Rather, this applies only when there is no greater Mitzvah to marry off one than another. Here, it is a bigger Mitzvah to marry off a girl in his Reshus.