To add to the discussion... according to the briefing, they believe that all of the technologies required for a human mission to Mars are 15 years away, assuming that all technologies are fully tested. This puts an approximate time-line near 2030 [1].

Question from Scott Powers, The Orlando Sentinel:

"The technologies you talked about that are still a long ways from being fully developed, the EDL [2], the propellant and communications. Do you have a sense for what kind of time tables we're talking about, to get those to the point where they'll be able to be used in manned missions?"

Answer from Dr. James Reuther [3]:

"[...] To get all the way to the point where we are able to put 10 metric tons, 10 times what we can do today, to the surface or greater, which is what's really required for a human mission, is probably going to take until the beginning of 2030 before we have all of those technologies in hand, ready to go at that scale. And, that means that they're fully demonstrated and ready for that kind of mission."

[1] https://www.youtube.com/watch?v=zBoj-1m-qLU#t=2304

[2] Entry, Descent and Landing

[3] http://www.nasa.gov/directorates/spacetech/about_us/bios/reu...