@selzero I already answered Jaws as the earliest movie that scared me, but I wanted to point out that I'm also one of those kids who freaked out at Gremlins in the theater (I made it to the scene where Billy discovers the teacher in the science lab before me and my younger brother started wailing in fear), which led to the creation of the PG-13 rating.
@Tim_Eagon@selzero For me, it was Forever Young, Forever Free. The rolling-downhill-in-a-tire scene was VERY frightening to me. It might even have influenced my sense of what is an acceptable level of risk.