From what I understand about Mayhew's knees - he had "wobbly" knees, meaning they wouldn't lock out properly, and that made it very difficult for him to walk. Similar to what Darth Avlectus stated, I've heard many cases where knee replacement absolutely changed the quality of life for people. Hopefully this is the case for Mayhew. Given a few months of rehab, he should be able to walk just fine. As for any crazy Tarzan stunts, I'd imagine they'd go with CGI anyway - whether it was Mayhew or not.
As for his age - I believe he's actually younger than Harrison Ford, right?