Prophet Muhammad was married with only with one woman until the age of 53. And it was a big thing that because all the Arabs were married with 5-6 women. He was first married with Khadija when he was 25. And Khadija died when his age was 50. After the death of Khadija, he married multiple women who were widows, divorcees and captives except Ayesha.

William Montgomery Watt (Scottish Orientalist, historian) says that all of prophet Muhammad's marriages had the political aspect of strengthening friendly relationships with the other Arabian tribes. 

Francis Edward Peters (Professor Emeritus of History, Religion and Middle Eastern and Islamic Studies at New York University) told that many of the marriages of the prophet were political and some compassionate. 

And according to John Victor Tolan (Historian of religious and cultural relations between the Arab and Latin-speaking civilizations of the Middle Ages.), the prophet's marriages were mainly attempted to make political alliances.

 According to Laura Veccia Vaglieri (Italian orientalist), Prophet Muhammad married only one woman throughout his youth. Although he lived in a society where plural marriage was a normal thing and divorce was so easy. He married a woman who was older than him and did not marry until her death when his age was 50. He married all of his wives except Khadija for social or political purposes. He wanted to honor the pious women, or wanted the loyalty of certain tribes so that Islam would spread amongst them. 

Many say that prophet Muhammad married Ayesha when she was 6 years old. That's actually not true. See the reason here.

Sources

  • “Muhammad’s Wives - Wikipedia.” Muhammad’s Wives - Wikipedia, en.wikipedia.org, 17 Sept. 2012, https://en.wikipedia.org/wiki/Muhammad%27s_wives.