Islam's most trusted sources, of course, tell a completely different story. For instance, here's a brief sample of narrations from Sahih al-Bukhari, Sahih Muslim, and Sunan Abu Dawud (Islam's three most respected hadith collections).
Sahih al-Bukhari 5158—Narrated Urwa: The Prophet wrote the (marriage contract) with Aisha while she was six years old and consummated his marriage with her while she was nine years old and she remained with him for nine years (i.e. till his death).
Sahih Muslim 3311—Aisha reported that Allah’s Apostle married her when she was seven years old, and she was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old.
Sunan Abu Dawud 2116—Aishah said: The Apostle of Allah married me when I was seven years old. (The narrator Sulaiman said: Or six years.) He had intercourse with me when I was nine years old.
Usually, when I hear a Muslim say that Aisha was older than nine when Muhammad had sex with her, I assume that the Muslim is simply ignorant of his sources (i.e., that the Muslim isn't trying to deceive anyone; he's merely ill-informed). Reza, however, claims to be an "internationally acclaimed scholar of religions" (even though he's an associate professor of creative writing). In carefully examining Islam's sources as research for his book, surely he must have come across dozens of narrations reporting Aisha's sexual relationship with Muhammad when she was only nine years old. The fact that Reza contradicts the unanimous testimony of his sources can mean only one thing: Reza is lying to his readers in an effort to make Muhammad seem better than he actually was.
Here's a recent discussion of Reza's claim (on ABN's program Jihad Watch):
For more on Muhammad's relationship with Aisha, click here.