The Notion that children were seen as adults at a very young age is plainly wrong.
https://archive.org/details/TheWorldWeHaveLost/page/n101/mode/2up Laslett, Peter The World We Have Lost (1965) famous historian, part of the Cambridge group.
“First of all there is the possibility that Shakespeare was deliberately writing a play about love and marriage amongst boys and girls. Scholars have discovered that he actually reduced Juliet's age; in the poem containing the plot for the play, Arthur Broke's Romeus and Juliet, published in 1562, Juliet was sixteen. Four years later another author who told the story, William Painter in his collection of novels The
Palace of Pleasure, made Juliet eighteen years old. When Shakespeare came to adapt it thirty years afterwards he may possibly have had to reduce the heroine's age to suit the boy actor who was to play her part”
casts more than enough doubt on the assumption.
The same historian did extensive archive research:
https://archive.org/details/TheWorldWeHaveLost/page/n97/mode/2up Laslett, Peter. The World We Have Lost (1965).
".So she had been married at twelve, or early thirteen, and all those other ladies of Verona also. Miranda was married in her fifteenth year in the Tempest. It all seems clear and consistent enough. The women in Shakespeare's plays, and so presumably the Englishwomen of Shakespeare's day, might marry in their early teens, or even before that, and very often did.....
Yet this is not true. We have examined every record we can find to test it and they all declare that, in Elizabethan and Jacobean England, marriage was rare at these early ages and not as common in the late teens as it is now. At twelve marriage as we understand it was virtually unknown."
Juliet is not a valid example.
The example from an internet Porn book is contrived and wrought, none of their main arguments rely on it, so it can easily have been a mistake.
Fact is that in the mid 1800s Marrage Age statistics were published (before 1836's law only Church of England marriages were reported on. )
https://archive.org/details/annualreportofre171854grea/page/n15/mode/2up Annual report of the Registrar-General of births, deaths and marriages in England. General Register Office. Government publishing marriage-statistics. look at the table at the bottom right:
1845: 119.539 newly-weds, , and 11.835 widowers/spinsters=131.374 first marriages for women.
Underage (i.e. requiring permission from parents because under 21: 19.376 14.75% were under 21 ).
The pattern is comparable for the rest of the columns with some variation per year.
Conclusion: About 85% of girls were 21 or older at first Marriage. Only 15% was 20 or younger. Teen marriages were NOT the norm.
"Marriages of minors.-9210 men and 28,797 women, or 38,007 men and women, married under 21 years of age ; so that the proportion of minors in 100 men who married was 5.77, in 100 women 18.03; " so for all marriages 18% were 20 or younger, were considered minors who required parental consent and 82% were 21 or older.
One may think that marriage patterns may have been different before 1800s: It has been researched extensively:
Laslett, Peter The World We Have Lost (1965) most common marriage age in the records was 22 with an average of 24. "We have examined a thousand licences containing the ages of the applicants, issued by the diocese of Canterbury between 1619 and 1660 to people marrying for the first time. One woman gave her age as 13, four as 15, twelve as 16: all the rest were
17 and over, and 966 of the women got married for the first time after the age of 19, that is nearly 85 per cent. The commonest age of first marriage for women in this sample was 22, and the median age -- the age below which as many got married as got married above it - was about 22.75: the average, mean age was about 24.
First Marriage age for women in the 1600s was 22-24 most commonly.
How the West 'Invented' Fertility Restriction, Nico Voigtländer & Hans-Joachim Voth 2011, https://www.nber.org/papers/w17314 argues: "Europeans restricted their fertility long before the Demographic Transition. By raising the marriage age of women and ensuring that a substantial proportion remained celibate, the "European Marriage Pattern" (EMP) reduced childbirths by up to one third between the 14th and 18th century." so it is a known historical fact that children did not marry. Women married late.
Aside from the UK this pattern is clearly shown in the USA https://www.nber.org/papers/h0080 shows it from 1730 onwards. The same conclusions from the mid1800s onwards are shared by http://users.hist.umn.edu/~ruggles/Articles/Fitch_and_Ruggles.pdf
Women married late teens/early twenties young marriages occasionally ocurred but were not the norm.
Another region in Europe confirms the picture https://www.rug.nl/staff/r.f.j.paping/ageatfirstmarriage.pdf 1720-1800 Marriages in the teens were not the norm. Mi-twenties was most common.
Don't forget that midwives and medics were well aware of the risks of teen-pregnancy/delivery.
https://archive.org/details/familyphysiciang00hamm/page/592/mode/2up The family physician and guide to health ... Including a treatise on midwifery and the diseases peculiar to women by Hammack, Elijah B Publication date 1869 Topics Medicine, Popular
Publisher St. Louis, Southwestern book and publishing co.
"It is worthy of remark that marriage at too early an age is not conducive to health or longevity, but, on the contrary, the mortality among young married persons, I mean of married persons under the age of twenty, particularly women, is very great. I do not think that women ought to marry under twenty-two, or men under twenty-five years of age.".
Before Muhammed and Aisha the Romans had census records, which have been used for research.
Bagnall, Roger S. The Demography of Roman Egypt. E-book, Cambridge [England]: Cambridge University Press, 1994, https://www.cambridge.org/core/books/demography-of-roman-egypt/marriage/EDC74C1C06DA145343594EA1D4001496
Use three centuries of Roman Census data (000-300) in Egypt to calculate age of first marriage.
The most common ages of first marriage appear to be roughly equally ascending from 14-23 and then starting to go down. 12-13 year olds were uncommon.
No clear evidence of early onset of menstruation and related low marriage age. Lower first marriage age than nowadays, but not spectacularly lower and fair numbers of 19-23 year olds.
Aristotle and Hippocrates wrote 1000 years before Muhammed and Aisha and were widely used in Persia and Arabia and wrote that a girls menstruation on average started at 14-15 but that healthy, strong girls therefore could start bearing children 15-17. So they were well aware of the risks to young girls' bodies if impregnated too early.