1,400 years ago, the Qur’an was revealed to mankind. Among the various covenants and guidance in the Book was an outline of marital relationships, abolishing marriage to underaged minors which was prevalent in ignorant and uncivilised cultures at the time.
The Qur’an says that they
must have sound judgment (4:6)
consent (male and female) of both parties (4:19), and
be able to participate in a binding contract (4:21).
Furthermore, the Qur’an instructs us to marry Mu’mins (2:221) who are defined as people who enjoin virtue and forbid vice (9:71) and prohibits us from marrying Mushriks (2:221)- traditionally translated as idolaters, but which has a wider meaning in the Qur’an, nominally those who forsake their intellect and conscience for supposed “scripture” (9:31, 24:55).
While no specific age is mentioned, as people reach mental maturity at different ages (some become capable at 18, others at 21 and others still, 40) these requirements exclude children from the people who are able to be married.
Sound Judgment
4:6 “Test (trial) the orphans (Arabic: wa-ibtalu l-yatama) until they reach the age of marriage (Arabic: balaghu l-nikaha); if you then find sound judgment in them, release their property to them; but consume it not wastefully, nor in haste against their growing up. If the guardian is well-off, let him claim no remuneration, but if he is poor, let him have for himself what is just and reasonable. When you release their property to them, take witnesses in their presence (Arabic: Fa-ashiddu alayhim) : But all-sufficient is God in taking account.”
This means that you need to test for this ability over and over, until you can be convinced that they have sound judgment – and only then they can be said to reach (balaghu) the age of marriage. Sound judgement and capability in managing finances means the person is now ready to go out and work, get married, and manage his or her own life financially.
Consent of Both Parties
4:19 O You who have chosen to be graced with belief! It is not lawful for you to force women into marrying or holding on to them in marriage against their will.
This is absolutely clear and indisputable, and requires no explanation.
Contractual Gravity
It’s interesting to note the gravity attached to the marriage contract - we see this being mentioned in 4:21
4:21 "And how could you take it away after you have given yourselves to one another, and she has received a most solemn pledge from you? (Arabic: Meethaqan Galezaan)?"
In fact, the phrase “Meethaqan Galezaan” used in the marriage contract is the same as the contract made between God and His Prophets.
33:7 And when We exacted a covenant from the prophets, and from thee (O Muhammad) and from Noah and Abraham and Moses and Jesus son of Mary. We took from them a solemn covenant (Arabic: Meethaqan Galezaan).
In most countries in the world today, the legal age for entering into a contract is 18 years of age. This age is usually arrived at by consensus of experts in all areas especially child psychology, criminology, legal etc, which is a method of decision-making (shura, or consensus) mandated in the Qur'an.
The character of the Prophet
The Qur’an also says that Prophet Muhammad was one of impeccable character
68:4 And thou (standest) on an exalted standard of character.
Therefore, it is inconceivable that the Prophet would have gone against the commandments in the Qur’an and married Ayesha at such a young age as mentioned in the hadith - unable to give her own consent, or manage her finances, or in a position to participate in a solemn covenant, and her age is irrelevant when considering this question of the age of marriage.
Other verses
In the surah concerning divorce, there is a verse that states "And those who no longer expect menstruation among your women - if you doubt, then their period is three months, and [also for] those who have not menstruated" (65:4)
Traditional interpreters have taken "those who have not menstruated" to mean children are also allowed to be married before they get their period. There are two issues with this interpretation:
It totally ignores the three verses mentioned above concerning marriage.
It also contradicts their assertion that girls are considered mature enough to marry when they get their menses.
This verse alone cannot cancel those other verses, so "those who have not menstruated" cannot mean a child who has not had menses, but it could be a woman with dysmenorrhoea. It's not that difficult to think about, unless one were to purposely pick and choose particular verses of the Qur'an to adhere to, to the exclusion of other verses.