However, there's no missing verb. I think it means something like this:
"For millions of earlier immigrants in transit from their own cultures to American society at an age when they were so young that a culture barely had time to create itself for them, the movies became the substitute acculturation process."
Another example of the problematic phrase:
"He came to America so young he couldn't remember Russia at all."
"He came to America when he was so young that he couldn't remember Russia at all."