^I think it's George's grey hair (which to his credit, he rarely dyes). Makes him look older, more distinguished. Downey looks kinda lived-in but has a boyish quality to him.
Anyway, I'm more thinking in terms of the sort of actors the studio might be looking at than my own preferences. There aren't too many actors out there who can topline a franchise and would suit this role. Christian Bale is too intense but Jake Gylenhall might fit the bill - though Prince of Persia would suggest he's not that big a draw. James Franco maybe? Both Oz The Great and Powerful and his Planet of the Apes movie put bums on seats, even if he wasn't necessarily the big draw there.
If they wanted someone of Cruise's mettle, the safest option might be to go colour-blind and cast Will Smith (I don't think the show has a big enough following to result in the internet meltdown that often accompanies such casting).