Christopher Beam and Jeremy Singer-Vine have turned Rotten Tomatoes' database into an interactive "Hollywood Career-o-Matic tool" that lets you "map the career of any major actor or director from the last 26 years." Above are the averages for actors and directors:
These trends seem to make sense. Most actors have to appear in good movies early in their career. Those who don't risk being flushed out of the business. Once they've established themselves with a good film or two, they can safely make some bad ones. But all in all, they don't have nearly as much control over film quality as directors do. Directors' scores spike over time, presumably because only the best ones stick around long enough to make so many films.
The Dish 