Records for this man don't go back much further than 1176 CE, right around the time of the Assassins' confrontation with the Saracen General, Saladin. I'd wager Al Mualim was in his late forties at the time, but it's hard to say. In any event, Al Mualim was an accomplished Assassin and feared leader.
Calm but commanding, he led his group of Assassins into one of its most fruitful periods, treating his pupils strictly but fairly, with patience, honesty and understanding. He had many admirers, among friends and enemies alike, and was looked upon as a father by many of his apprentices - the great Altaïr Ibn La'ahad most notably.
Considering all this, Al Mualim should have attained a legendary stature were it not for his rapid decline into corruption around 1191. After he got his hands on that First Civilization artifact - the Apple of Eden - he was no better than the Templars, and eventually succumbed to their way of thinking.
Within months of acquiring it, he abandoned the Creed and began blathering on about peace and order being achievable only through coercion and manipulation and all that. Cynical, obviously, but appealing too.
That's the trouble with the Templars in general, I think. Their philosophy is just so damn simple, so tantalizing, offering easy solutions to complex problems. Just do X, Y, and Z and all your problems disappear. That's what the Apple was for Al Mualim. An easy solution to all the chaos he saw in the world.