The Islamic teachings are very clear. Islam allows for marriage of consenting, physically mature men and women: for marriage both the man and the woman have to be of marriageable age and able to give their consent freely. The Holy Quran says that women must not be taken into marriage against their will.
O ye who believe! it is not lawful for you to inherit women against their will; 
This is simple and very straightforward and if in some countries specific age limits are in place then those must be respected and followed.
Narratives related to the time of the Prophet Muhammad(sa) show that Hazrat Ayesha(ra) for sure was not six years old at the time of her marriage but rather was 12-13 years of age, an age legally acceptable for marriage in this modern age even in the developed countries.
Contemporary critics have repeatedly claimed, without merit, that Prophet Muhammad(sa) married Hazrat Ayesha(ra) when she was not of age, some asserting as young as six. The facts instead indicate that Hazrat Ayesha(ra) was likely around the age of fifteen at the time of her willing marriage with parental consent, and she may have been as old as nineteen or twenty. A variety of authentic historical references substantiate this conclusion.
Before delving into Islamic history, it is necessary to consider that the age deemed “acceptable” to marry is not some objective standard across time, culture, and religion—but a subjective standard based on social construct. For example, the Catholic Encyclopedia reports regarding Mary, mother of Jesus:
…the priests announced through Judea that they wished to find in Juda a respectable man to espouse Mary, then twelve to fourteen years of age. Joseph, who was at the time ninety years old, went up to Jerusalem among the candidates; a miracle manifested the choice God had made of Joseph, and two years later the Annunciation took place. 
Likewise, the Talmud recommends “marrying off one’s daughter as soon after she reaches adulthood as possible, even to one’s Slave.”  In fact, the Talmud presents some peculiar guidance on marriage, also stating, “A maiden aged three years and a day may be acquired in marriage by coition, and if her deceased husband’s brother cohabits with her, she becomes his.”  So while the Catholic Encyclopedia tells us that Mary and Joseph were married at the ages of about twelve to fourteen and ninety, respectively, the Talmud permits marrying girls as young as three years and one day. Lest this seem like an attack on Christianity and Judaism—which it is not—consider marriage laws in the West.
For centuries in Scotland, the age of consent for girls was twelve—and parental consent was unnecessary.  Only in 1929 was the age raised to sixteen for girls.  In America even today, Hazrat Ayesha’s consenting marriage to Prophet Muhammad would be considered valid. For example, in New Hampshire, the legal age for girls is thirteen with parental consent.  In Massachusetts, the legal age for girls is twelve with parental consent.  In Mississippi, there is no age minimum for girls, as long as there is parental consent.  In California, there is no age minimum for girls, as long as there is parental consent. 
Granted, the American state laws were passed in the nineteenth and twentieth centuries—not today. And granted, Americans aged twelve or thirteen do not typically get married today. The point is, however, that even in recent American history, Americans found value—not objection—in girls marrying at twelve or thirteen (or even younger), and passed laws through their respective state legislatures to affirm that value. This exercise in no way argues for a return to such marrying ages. Rather, it merely establishes the point that “appropriate” marriage ages are based on ever-changing social constructs—not some supposed objective, advanced contemporary standard. Therefore, if we are to accuse Prophet Muhammad(sa) of any impropriety in marrying Hazrat Ayesha(ra) even if she was fifteen, then we must also condemn the Old Testament, New Testament, Europe, and numerous American states. Perhaps most unknown and shocking about American marriage laws is that child marriage is still legal in America—accordingly there have been over 200,000 child marriages in America since the year 2000, with some children as young as 11.  Fascinating how critics of Prophet Muhammad(sa) cite inconsistent information to allege him of wrongdoing while remaining silent on this documented epidemic of child marriage in America. Hazrat Ayesha’s marriage to Prophet Muhammad(sa) was not by any means unusual when compared across time and culture—even to contemporary standards. Moreover, the assertion that Hazrat Ayesha(ra) was only six is meritless.
In determining hadith authenticity, it is necessary to consider how many different narrators can relate a hadith back to the original source—be that source Prophet Muhammad(sa), Hazrat Ayesha(ra), or a companion. The more consistent narrators exist and the more in agreement they are with one another—the more authentic we may consider the hadith.
There exist only a few narrations of the age of Hazrat Ayesha’s marriage at age six, while many more verified and authentic references indicate she was in her mid to late teens. Several historical events and ahadith narrations demonstrate that Hazrat Ayesha(ra) was likely fifteen or sixteen or as old as nineteen or twenty at the time of her consenting marriage to Prophet Muhammad(sa). Critics ignores each of these. Accordingly, here are just three of the many arguments that affirm Ayesha’s age as between fifteen and twenty.
First, Hazrat Ayesha(ra) was the daughter of Hazrat Abu Bakr(ra). Tabari reports, “All four of [Abu Bakr’s] children were born of his two wives…during the pre-Islamic period [i.e., pre-610 AD].”  Hazrat Ayesha’s marriage to Prophet Muhammad(sa) took place one year after Hijra (emigration to Medina), or around 624 AD. Therefore, even if Hazrat Ayesha had been born as late as 609 AD, only a year before Prophet Muhammad(sa) claimed prophethood, she would have been roughly fourteen at the time of emigration to Medina in 623 and therefore no less than fifteen at the time of her marriage to Prophet Muhammad(sa). This is a far cry from the age of six that critics assert.
Likewise, most historians report that Hazrat Asma(ra), Hazrat Ayesha’s elder sister, was ten years her senior.  The books Tahzibut Tahzib and Al-Bidaayah wa an-Nihayah both report that Hazrat Asma died at the age of one hundred, in 73 AH (695 AD).  This means that Hazrat Asma(ra) must have been no younger than twenty-seven at the time of emigration. Hazrat Ayesha’s marriage to Prophet Muhammad(sa) was in 1 AH or by some sources 2 AH, when Asma was twenty-eight. This means that at a minimum, Hazrat Ayesha(ra) was eighteen or nineteen upon her consenting marriage to Prophet Muhammad(sa).
Finally, it is interesting to note that none of the opponents of Prophet Muhammad(sa) objected to this marriage during or after his lifetime. Such individuals, who wasted no excuse to oppose and malign the Prophet, remained completely silent on this allegedly inappropriate marriage. How could that be? The only logical explanation is that there was nothing objectionable about this marriage because it was between two mature partners who both consented. It is simply ludicrous to assume that critics 1400 years after Prophet Muhammad(sa) are a better judge of his character and acts than were his contemporaries. Prophet Muhammad(sa) himself made this argument when he recited the Qur’an,
“I have indeed lived among you a whole lifetime before this. Will you not then understand?” 
In this verse, Prophet Muhammad’s critics are reminded to reflect upon his entire life and cite a single flaw in his character, a single injustice he committed, or a single lie that he told. History records that not a single contemporary—friend or foe, ally or adversary—could cite a flaw in Muhammad’s life prior to or after his claim to prophethood. Accordingly, history records that any criticism he received after his claim to prophethood was not of his character or acts, but of the claim to prophethood itself. The argument was an open statement that Prophet Muhammad(sa) lived a demonstrably flawless and truthful life—so much so that even those who sought to kill him for his claim to prophethood, could not find flaw with his morals or behaviors.
The above examples are not exhaustive, but clarify evidence that critics ignore. Several other authentic ahadith and well-recorded events discredit critic’s allegation that Prophet Muhammad(sa) married Hazrat Ayesha(ra) when she was underage. The examples presented, however, should more than suffice that Hazrat Ayesha(ra) was instead a mature person who married Prophet Muhammad(sa) of her own free will.