Too old for the parts if you want them to look like they did when they finished the last movie they did.  Have you seen Harrison Ford lately--no miracle of plastic surgery could take that many years off him.  He was no spring chicken when he made the original film.  They certainly are not too old to play older versions of the characters--that is if they agreed to do so--I think that Carrie Fisher has a love/hate relationship to the Princess Leia role and I think she would rather leave all that behind her and concentrate on her shock treatments and writing.