Altair was born into the Assassin Brotherhood, in the stronghold of Masyaf. His early life wasn't a happy one - his mother died during childbirth. Then, when he was a young boy, his father was killed during the first siege of Masyaf. Only shortly thereafter the assassin he died to save committed suicide in front of the 11-year old Altair
With his parents gone, Altair looked to Al Mualim - the then Mentor of the order - as a father figure. Al Mualim recognized Altair's potential, and took on his training personally. Altair reached rank of Master Assassin by the age of 25 - an unheard of accomplishment. If there was an Assassin Book of Records, there'd probably be a picture of this fella on the front.
Altair was one of the best fighters in Assassin history - with an arrogance to match. After a disastrous mission in 1191 in which he broke the creed and very nearly let an Apple of Eden fall into Templar hands, Altair was sent back down to novice rank and forced to begin again.
As part of his rehabilitation, he was responsible for taking out the major players in the Templar order at the time.