The starting age of Marriage:
It is a Mitzvah upon every man to marry a woman at 18 years of age. [The Rebbe strongly encouraged this approach. Some Poskim rule this means that one should get married as soon as he begins his 18th year, which is after turning 17 years of age.] One who precedes and marries at 13 years of age [has fulfilled] the Mitzvah Min Hamuvchar. [However, in today’s times, one is not to get married this young.]
Child marriages-Marrying prior to 13 years old: Prior to 13 years of age, a boy may not get married, as it is considered like promiscuity. [This implies that a child may not get married until he has reached his 13th birthday. However, some Poskim rule a child may get married starting from 12 years old and onwards.]
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 Michaber ibid; Kiddushin 29b; Pesachim 113/1; Mishneh Pirkei Avos end of chapter 5/22; Rambam Ishus 15/2 writes from 17 years of age
The reason: Although in general, a person must fulfill all the Mitzvos as soon as he turns 13 years of age, nevertheless, the Sages received that regarding the Mitzvah of Peri Urevu it only begins at age 18. The reason for this is because one is required to learn Torah, and one only has time to properly do so prior to marriage. [See Michaber Y.D. 246/2] Now, the Torah learning of the Talmud begins from age 15, as stated in the Mishneh in Avos, [and therefore the Sages gave 2-3 years of learning the Talmud prior to marriage]. [Chelkas Mechokeik 1/2; Beis Shmuel 1/3; See Admur Hilchos Talmud Torah 3/1] The reason why the Sages stated one should get married at 18, even though it takes five years to complete Talmudic studies, is because it is possible to learn after marriage without too many distractions for about 2-3 years, prior to having many children. [Admur ibid]
 Igros Kodesh 13/23; 18/436; 20/175 “If only that the Ashkenazim would also get married at a very young age”; 23/221-222; 29/272; Letter 5,974 “You should be Makpid on the teaching in the Mishneh of 18 years old for marriage, as certainly the Mishneh is reliable and it applies to all types of Jews”; On the other hand, see Igros Kodesh 10/202; 28/341 [printed in Shulchan Menachem 6/95] where the Rebbe seems to resolve with the custom of getting married in the early 20’s and not 18 or even 20 years old.
 Beis Shmuel 1/3; Taz 1/3 in name of Maagid Mishneh; Biur Hagoleh 1/3 in name of Rambam; Maggid Mishneh on Rambam 15/1; Rambam Ishus 15/2 writes from 17 years of age; Aruch Hashulchan 1/11
Other opinions: Some Poskim rule one is to get married after completing his 18th year, which is after his 18th birthday. [Bach 1, brought in Taz 1/3]
 Michaber ibid; Yevamos 62b; Sanhedrin 77b; Rav Chisda in Kiddushin 29b
The reason: As he quashes his lusts and passions from the Yetzer Hara, and is like sticking a arrow into his eyes. [Kiddushin 30a; Rambam Issurei Biyah 21/25]
 Birkeiy Yosef 1 in name of Rabbeinu Yona in his Tzava, brought in Pischeiy Teshuvah 1/2; Aruch Hashulchan 1/11; See Igros Kodesh 29/272
The reason: As the people are weaker [i.e. less mature] and the generations have changed [and are thus unable to handle marriage at such a young age]. [ibid]
 Michaber ibid; Tur 1; Rambam Issurei Biya 21
 Michaber ibid; Tur 1; Bach 1; Rav Akiva Eiger that so is implied from Rashba 803 and 1181; Aruch Hashulchan 1/11; See Taz 1/3 that this means that it appears the relationship is for the sake of promiscuity, even though in truth it is not promiscuity, as they doing so in a way of marriage.
Other opinions: Some Poskim rule it is not considered like promiscuity. [Beis Shmuel 1/4; Tosafus Sanhedrin 77b; Rosh; See Beir Heiytiv 1/5 in anme of Kneses Yechezkal 55; Pischeiy teshuvah 1/3]
 Chelkas Mechokeik 1/3; Beis Shmuel 1/3; Beis Yosef 1; Implication of Rambam and Tur ibid;
 Bach 1 if father marries him off; Rashi and Tosafus on Sanhedrin 77b “A year or half a year before his Bar Mitzvah”; Some Achronim in interpretation of Rambam and Tur, brought in Chelkas Mechokeik ibid; Bach 1, brought in Chelkas Mechokeik and Beis Shmuel ibid, rules that only the father may marry him off from age 12, and the Sages instituted a Rabbinical Kiddushin for him at this age, and his wife needs a Get!