Not sure why people think the "Old Master" is Darth Maul. He was never the master. He was Darth Sidious apprentice. The "Old Master" is Darth Plagueis whom discovered the secret of immortality.
http://starwars.wikia.com/wiki/Darth_Plagueis
Maul was master to Savage Oppress during the Clone Wars, but that's beside the point.
Did you ever hear the Tragedy of Darth Plagueis the Wise? I thought not. It's not a story the Jedi would tell you. It's a Sith legend. Darth Plagueis was a Dark Lord of the Sith so powerful and so wise, he could use the Force to influence the midichlorians to create...life. He had such a knowledge of the Dark Side, he could even keep the ones he cared about...from dying. He became so powerful, the only thing he was afraid of was losing his power... which, eventually of course, he did. Unfortunately, he taught his apprentice everything he knew. Then his apprentice killed him in his sleep. Ironic. He could save others from death... but not himself.
Plagueis is dead. It's right there in the text. There's nothing there that suggests that he discovered the secret of immortality. Quite the opposite, in fact. Palpatine goes out of his way to note that he could save others from death but not himself.
I'm 100% certain that Lucas did this deliberately to contrast with the revelation at the end of the movie that Qui-Gon has discovered a way to retain his consciousness within the Force... to become immortal, effectively, through embracing the Living Force and balance within it, which is what Qui-Gon represented in The Phantom Menace , and what Yoda and Obi-Wan learn.*
I'm constantly confused by people's desire to see Darth Plagueis – a minor character at best who was mentioned once, in the dialogue above – come to prominence in the saga. But hey, different strokes for different folks.