MUSLIMS: did mohammed marry a 9 year old girl?

Actually, they were married when she was six, and she was admitted to his home at the age of nine. He was in his fifties. I don't understand the mentality of people like that either. Unfortunately, some Islamic countries do still continue this practice. There are various reasons that people might do this. But since Mohammed did marry Aisha when she was only six, it is permitted to marry girls off at that age. Still don't understand it myself, but then again, there's plently of crazy people in this world.