Muhammad bin Qasim was born around 695, his father died when he was young, so his education was handled by his mother. One of his close relatives was the Umayyad governor Hajjaj bin Yousef who was instrumental in teaching Muhammad bin Qasim about warfare and governing.
With Hajjaj's patronage, he was made governor of Persia where he put down a rebellion. At the age of seventeen, he was sent by caliph al-Walid I to lead an army into Sind. The Umayyad reasoning for this attack was to rescue some pilgrims that were taken captive by Hindu pirates.
Bin Qasim was sucessful, rapidly taking all of Sind and moving into southern Punjab up to Multan. The forces of Muhammad bin Qasim defeated Raja Dahar, and took his daughters captive (they were sent to Damascus). Following his victories, he was instructed by bin Yousef to allow the religious minorities to go about their way and not to harass them. Bin Qasim obliged. He also began preperations for an attack on Rajasthan. In the interm though, Hajjaj bin Youseff died, as did the caliph al-Walid I. The new caliph, Suleiman was a political enemy of Hajjaj and recalled Muhammad bin Qasim using the claims of Raja Dahar's daughters as a pretense (they claimed that bin Qasim had not treated them right - the validity of this claim is questionable). Bin Qasim was wrapped in oxen hides and returned to Syria. He could have very well not followed the caliphs order, but he did. Muhammad bin Qasim would die in jail, at the age of twenty.
Bin Qasim's rapid gains at such a young age have led many scholars to speculate on how much he could have acheived had he been given the chance. Some have written that he may have taken all of South Asia, but whether or not this is valid is debatable.