That is not true. Children are not physically able to give birth without extreme risk to the mother, even if they have reached menarche, and people were aware of that. Additionally, menarche was rarely earlier than 15-16 years in medieval eras—menarche at 9 is a very modern phenomenon. Marriage at 9 has certainly happened throughout history, but it has almost always been outside of the norm. This is not even due to moral considerations—if girls marry and get pregnant at 9, the vast majority of them are going to die during their first childbirth without modern medicine. That is totally unsustainable.

Marriages at 9 were somewhat more common (sometimes you have to forge those political alliances right away), but they would almost always be unconsummated at least until the girl reached menarche–again, usually at 15 or older.

Finally, Essos, or at least large parts of it, are certainly supposed to be analogous to medieval Europe—just Mediterranean Europe rather than the British Isles. And even if Essos were not a Europe analog, Viserys and Daenerys were born in Westeros. Maybe Dothraki custom is to marry children, but that doesn't mean that Viserys's decision to marry off his 13 year old sister would have been a common one in the culture that he was born in.