Very early child marriage seems to have gone somewhat out of fashion amongst the British royalty and nobility after the 15th century, due to the example of Margaret Beaufort, mother to Henry VII and grandmother to Henry VIII, who was married off at 12, and pregnant at 13. The birth was very difficult and she never had another child after him. Both then and now, her subsequent lifelong infertility was attributed to damage from the birth, which showed anyone with the wit to listen, that sooner wasn't necessarily better, when it came to getting daughters started on dynastic marriages and heirs.
I remember that when Antonia Fraser published her biography of Henry VIII's wives, she had found contemporary letters from family members to other relatives, counselling against early marriages of their heiresses, because they did not trust particular kings and princes not to consummate the marriages immediately. I don't have the book here, but I recall that one letter writer warned that the girl concerned would "take harm from it", should her betrothed have sex with her following the marriage.