That movie with Robin Williams was called Jack--- kind of a bastardization of progeria, in my opinion.
That said, Neverending Story was a big one for me... I've even been to the studios in Munich, Germany, where it was filmed, and saw the original Falcor model. My absolute favorite movie as a kid was a little-known Canadian film called The Peanut Butter Solution... I watched it well over a hundred times around ages 8-10, and just recently found a pirated copy on eBay... one of the weirdest, coolest films I've ever seen. The soundtrack was done by Celine Dion, way back in 1985 before anyone had any idea who she was. Crazy.
The movie that I think had the biggest impact on me, however, was Stand By Me. I saw it when I was 10, and never forgot it. I can attribute my fascination with railroad tracks to that movie, and I always wanted to be those boys on their journey to self-discovery. Incredible film, and every time I watch it again it re-instills in me the fact that Stephen King, in his early days, was an absolute genius.
SD