That is still young in any planned series with no end in sight. The original Star Wars--once it was a success-- allowed Lucas to finish what he intended, and remember, in that era, films of that scale took more time to complete, so its not really comparable to modern film production.
Regarding the MCU, yeah, by the time of the first Avengers, it was a young series, with only the first two Iron Man films, The Incredible Hulk, Thor, and Captain America: The First Avenger in the can. 6 films in a series meant to have no predicted end is young, just like the DCEU at this point.