Dr. Ahmad Al-Mu'bi: Marriage is actually two things: First we are talking about the marriage contract itself. This is one thing, while consummating the marriage – having sex with the wife for the first time – is another thing. There is no minimal age for entering marriage. You can have a marriage contract even with a one-year-old girl, not to mention a girl of nine, seven, or eight. This is merely a contract [indicating] consent. The guardian in such a case must be the father, because the father's opinion is obligatory. Thus, the girl becomes a wife ... But is the girl ready for sex or not? What is the appropriate age for having sex for the first time? This varies according to environment and traditions. In Yemen, girls are married off at nine, ten, eleven, eight, or thirteen, while in other countries, they are married off at 16. Some countries have legislated laws forbidding having sex before the girl is eighteen." ...

The Prophet Muhammad is the model we follow. He took 'Aisha to be his wife when she was six, but he had sex with her only when she was nine.