According to what I've read, Muhammad did marry three women and the youngest one was Aisha. She was promised to him when she was a young child (something around 6 years old) and was married at age 9. She was around 18 or 20 years old when Muhammad died and served an important role in the development and spread of Islam in the 45 years that followed until her death. She died at around 65 years old which is fairly uncommon for women to die at such age at that time due to a high level of childbirth deaths and injuries. Of course there is no precise date for her birth or her marriage and there is much debate about it. From my preliminay research it seems that a majority of scholars support the idea that she was about 10 years old when she married though it's also commonly accepted that the union wasn't made "official" (wink-wink) until a few years later. She was still very young when her husband died.
I understood they married when Aisha was six although the marriage was not consummated until she was nine. But as you say,there is considerable debate (and no facts)