I've seen this topic repeatedly..and it's interesting that it comes up as often as it does... We should also probably be aware of the condition of marriage and relations between men and women that obtain in our own countries before setting ourselves up as a judge of people who lived over five centuries ago..
And in the case of Muhammad it's interesting to discuss His first marriage to Khadijih:
Khadijih was a good deal older than Muhammad according to tradition she was forty years of age...and note it was a twenty year marriage ..a monogamous marriage which was rare at the time.. The marriages that occurred later were for several reasons..such as affairs of state,
The age of Aisha as six or eight years is questionable...
(1) The tradition itself was composed well after a century of the supposed event...
(2) There was no standard calendar in use among Muslims until under the caliphate of Omar...People didn't observe birthdays and we don't know for sure how old these people really were.. IF Khadijih was really forty years of age when she married Muhammad and she had three children in the marriage she would be quite unusual..even today with medical science as we have it this would be unusual.
(3) There is a tradition that is often overlooked that has Aisha well past puberty when her marriage supposed to have occurred.. I'll cite it here:
Volume 1, Book 8, Number 465:
Narrated 'Aisha:
(the wife of the Prophet) I had seen my parents
following Islam since I attained the age of
puberty. Not a day passed but the Prophet
visited us, both in the mornings and evenings. My
father Abii Bakr thought of building a mosque in
the courtyard of his house and he did so. He
used to pray and recite the Qur'án in it. The
pagan women and their children used to stand by
him and look at him with surprise. Abu Bakr was
a Softhearted person and could not help weeping
while reciting the Quran. The chiefs of the
Quraish pagans became afraid of that (i.e. that
their children and women might be affected by
the recitation of Quran)."
(Hadith, Bukhari Vol 1)
The above tradition suggests that Aisha had reached the age of puberty while she still lived with her parents before the marriage with the Prophet. Since Muhammad left Mecca in 622 A.D. and all accounts agree that the marriage was not consummated until two years after the hijra, around 624 A.D. Aisha must have been older than supposed.
(4) There is also the prejudiced attitudes toward Muhammad and Islam that have been stirring in the West for centuries..