The tricky thing from the 70's/early 80's is that they were able to appeal to both markets. A movie like Star Wars worked for kids, but worked for adults/teenagers too, LOTR isn't trilogy I am going to show my 5 year old nephew.
I always pose this question about todays movies: How different would Jaws be if it were made in 2008? It would not be that adult, because they would be afraid of losing a market of young kids, you would have a guy like George Clooney or Brad Pitt be the star instead of a talented actor who isn't the 'sexiest man alive' like Roy Scheider. You would have dumb humor throughout, like bad one liners that hurt the movies seriousness. The director woudn't have the guts to show a little kid get eaten by a shark, not that I enjoy that moment in the movie, but it hits you at that point that the shark is a terror. And finally the movie would have a CG shark that would be transparent in 10 years and look.......like CG.
Jaws would be different in 2008, cause times are different, that's all I am saying. Every era I believe has a certain type of movie that succeeds: The 30's/40's had the sweeping lovestories like Gone With the Wind & Casablanca, The 50's/60's had the great epic movies like Ben Hur, Spartacus, Lawrence of Arabia, the early 70's were gritty with The French Connection, Serpico, The Godfather, and the late 70's/early 80's had the 'summer blockbuster' type movies like Star Wars, Jaws, Raiders. I just happened to grow up in that time period, but those others ones I named still have great movies from their era.