When he played different characters, they really were different characters. I've always wondered what his real age was back then. Well, not always, because as a kid, I took it for granted that he was Prof. Stokes' age, presumably 60 or 55. Apparently, early 40s, which makes sense, midway between Elliot and Ben. It's the difference between those two that strikes me now usually. He's not just convincing at both ages, he's convincing as both personalities. He had respect for the Ben character, where many wouldn't and might have played him without giving it much attention, while waiting for a more 'attractive' part. He threw himself into the mindset and soul of him.
This makes the appearance of Elliot Stokes that much more meaningful. They don't even pause to deal with it much in the dialogue, but his existence alone puts a very life-affirming and encouraging backstory in the viewer's mind. Naive, 'slow', 'criminal' Ben later educated himself, learned to think for himself and question what he saw around him, and his descendant is a professor who can run rings around everyone logically.
Along with the Trasks, who showed the kind of hypocrisy that was being questioned then, the Ben-Elliot story represents the best new thinking (or new priorities) that we saw entering the popular culture in the '60s.
