No, it would be theft of services if they sneak into a theatre. On the other hand, if they stay at home and watch a bootleg DVD they bought from a criminal because they were refused admission due to their age, or because they want to watch films the cinema doesn't show, then it's no longer "theft of services", since they are no longer taking up a seat in the cinema. The "service" that is stolen is not the film itself but the delivery mechanism, much as you can steal a DVD from a shop.