It's worth remembering that the first two Doctors were both older men, and they both had younger male companions to handle the action and sex-appeal stuff -- Ian, Steven, and Ben for the First Doctor, Ben and Jamie for the Second. (And to some extent Mike Yates for the Third, though he could handle his own action well enough. And Harry Sullivan was introduced when it was thought the Fourth Doctor would be less physical and would need an action sidekick, but it proved unnecessary.) And even Eleven had both Amy and Rory as companions. So if they did cast an older Doctor, they could always give him a younger male companion again.