Mohamed married a six-year-old and had sex with her when she was nine. Is sex with a four-year-old allowed in Islam?

As for your women past the age of menstruation, in case you do not know, their waiting period is three months, and those who have not menstruated as well. As for those who are pregnant, their waiting period ends with delivery.1 And whoever is mindful of Allah, He will make their matters easy for them. - From Quran

“those who have not menstruated as well” seems to indicate no lower age limit. The Dawah crowd on YouTube defends sex with four-year-olds.

What is your opinion?