Given the incredible recent advances in rocket propulsion and other space technologies it is feasible that we will make the breakthroughs necessary to allow human beings to land on Mars within the next six years. However, it is more likely that a mission to Mars will take a little longer. Chemical fuel used to power rockets is fairly inefficient and unless a new cutting edge technology is developed fully in the next couple of years, Musk may have to put his plans on hold.
It's an interesting and ambitions plan, but who's going to pay for it? Elon Musk is certainly creative, but if it costs 10billion to send one person to Mars, only a very select few could afford to do it. I get that the planet is crowded and the availability of resources will be an issue, but his timeline doesn't seem very realistic.
Elon Musk is being too optimistic, maybe to capture the public's imagination and to interest investors. His last shuttle exploded so he needs to go back to the drawing board. Mars is so far away, if something goes wrong, people will die. Such a journey should not be rushed, so when Elon Musk says he can do it in six years, he is being foolish.
I do not believe that anyone could come up with a plan to actively sustain life on another planet. Humans have walked this earth for billions of years and if humans were meant to be living on Mars as well than we would have already been living on Mars. The Bible talks about life in Heaven and on Earth. Not in Heaven and on Earth and on Mars.