In a book sain in the Qur'an written by nadiah thayyarah xplains, among these serrations there is a soft cartilage that strengthens it. The soft cartilage will gradually get stronger over time.
The book 'Science in the Qur'an' written by Nadiah Thayyarah reveals the maturity and maturity of a person when he has reached the maturity of the mind, that is when the head has reached a certain shape and is no longer enlarged or shrinking.
When humans reach the age of 40 years, this cartilage that we know with the brain becomes more perfect level of violence. It will grow and will refine the skull structure so that it affects the mind core located in the brain.
Therefore prophecy came to the prophets when they were 40 years old.