I hope they get mature actors to play the roles, as the average age of the crew of the Ares is 46, and they only get older from then on in. If it was a film I'd speculate on casting but I don't know the names of many American TV actors.
I am within an hour of the end of Green Mars on audio, and I'm enjoying it even more than when I read the book. It's very exciting. I said at the start of Red Mars that the narrator, Richard Ferrone, wasn't using much voicework to indicate the ages, nationalities and genders of the main protagonists. Which was a fair enough choice. But over the sixty or so hours he has put in for Red and Green Mars this has gradually broken down, and now he's acting Sax with brain-damaged aphasia and Maya as a vigorous and scary old woman, and Desmond with a slight Trinidad accent and so on. Very enjoyable. I'm afraid Blue Mars might disappoint me slightly but Red and Green have been great. Surely his master work.