Bill Nye is in his late 50's, but if he holds up physically, I think he'll surprise a lot of people. He was at least into his 40's an excellent cyclist -- and may still be -- and in Seattle, that means uphill in both directions.
He'd always wanted to go on a space mission, and although he was on a PBS special about space flights, I think NASA lost a huge opportunity by having John Glenn be their token big name. He could have gotten so many kids interested in science if they had sent him.
He's by far my favorite going in.