Yes, I know all kids are deifferent, but would you reckon this is a bit too scary to show an almost 12 yr old or not ?
Depends on the 12 year old. Me at 12 would have loved it but if it's a sensitive flower that loves Hannah Montanta - perhaps...
Well, the movie's from 1982 so your 12-yr-old will probably be able to see right through the bad special effects.

True, but if the 12 year old has the same imagination as I had at that age, you may scar them for life.
At almost thirty, I still can't watch The Neverending Story, Ghostbusters or The Labyrinth.....
Poltergeist is far above and beoynd the three movies you mentionned though as far as sheer terror is concerned. Isn't Neverending Story a PG movie?