imagine you working on a research and you know you might probably live only for the next 10 years or so. You want to try and get someone up to speed on your research. Ideally you uld like someone young so that he/she has enough years to give time on that research... now going back to Prophet Muhammad (p.b.u.h) he was given the message from the true God of whom to worship, how to worship, how to lead life on this earth. He wanted this knowledge and this work to be continued for long time after his departure from earth. It was more than obvious he wanted someone who had many many years ahead of her to carry th emessage forward. Also only a spouse would know the minor details of her husband over day and night. That was the reason for that marriage. Although he married her at 6 yrs the marriage only came into existence when Aisha (r.a) became 9 yrs. Until then she was at her parents. If you do a bit of reading you would know the facts.
