At age 16, an undersized army brat landed a part as a 12-year-old on a Canadian television show. Confident he could make it in the U.S., he moved into a tiny apartment in the slums of Beverly Hills. Three years later, he was struggling to scrape by and ready to retreat. But then came his breakout roles — Alex P. Keaton on the sitcom Family Ties, and Marty McFly in the Back to t...