About eight years ago, something happened to Nicolas Cage. Known until then as an offbeat but fascinating charactor actor, Cage won an Oscar for playing an alcoholic who literally drinks himself to death in Leaving Las Vegas — and then he suddenly turned into an action hero. Many, but not all, of Cage’s onscreen adventures since then have been produced by Jerry Bruckheimer, including the R-rated action movies The Rock and Con Air and the PG-13 heist flick Gone in 60 Seconds. But now, with National Treasure — a PG-rated romp through the relics, myths and legends surrounding America’s founding fathers — Cage and Bruckheimer have set their sights on a younger, and perhaps less critical, audience. This film is being released as a full-fledged Walt Disney picture, and unlike some of Cage’s more intense flicks, this one has the benign villains and occasionally silly sensibility of those films Disney used to churn out back in the ’70s and show as two-parters on their Sunday-night “Wonderful World of Disney.” The main difference is, this new film has bigger stars and a bigger budget.