Originally, Lister was meant to be an older character. Now Craig Charles is the age of that original Lister. Besides, the characters of Red Dwarf have never been the most skilled, fittest high-flying members of the crew like in many other programme. Rimmer and Lister are the lowest ranking crew-members, one a lazy (if affable) bum, the other... well, he has issues. And that's not getting into the other quirky characters. If anything I think the ageing will just add to the comedy, if it's even a factor.