I believe that it wasn't that long ago that girls as young as 14 were married in Europe and the United States. I personally met a couple where the girl married when she was 15. But 14-15, while under the modern age of consent in most states, is still after the age of puberty.
Mohammed married this girl when she was six, so she had no choice about marrying him. She was a slave, not a real wife. And he had sex with her when she was nine, an age when sex doesn't even make sense to most girls. She probably didn't even know what he was doing to her. But it must have been very painful, and she must have cried afterwards, because she was raped. Nine-year-old girls cannot consent to sexual intercourse, they lack the capacity.