re: Parenting question: approximately what age would should kids first watch these movies?

Maturity level is more of a factor than age.

The kid should be able to identify the difference between fantasy and reality. Important for your sanity, will the movies cause them to have nightmares and come running to your bed.

Start them off with Ghostbusters, Star Wars, Marvel movies, then move to Jurassic Park and Lord of the Rings. I'd save Terminator and Alien for a while - see how they react to the others.

Gremilins, E.T., and countless other 80s & 90s movies can be added to this.

My son handled all the sci-fi movies well, and loves World War 2 history. So this past summer, at 12, we watched Saving Private Ryan, Band of Brothers and The Pacific together.