@z0z0,
.. and so was everyone else who lived before the 1850's or so - the term itself didn't exist prior to 1886.
Muhammad, 52, followed an Arab custom in marrying Aisha, 9, who, according to the custom, was deemed an adult because she had had her menstural cycles. She was 6 when Muhammad, 49, engaged her. At that time, she was already engaged to another man, but that engagement was dissolved by mutual consent.
As Muhammad aged, the revolutionary within him died out with his youth, and he resorted to some pre-Islamic traditions, such as his marriage to Aisha (his only virgin bride), the Islamization of hajj, and reverance for The Black Stone (a meteriod) of Kaaba.
Even today, the age of consent isn't 18 everywhere, even in every state of the US. In Denmark and Austria, it's 14. For most of Europe, it is 16.