The word "youth" can be either singular when it refers to one single person, or plural when it refers to a group of people. Both "look" and "looks" are correct without context, and therefore both are accepted. You only see one, but the others exist already.

• http://www.oxforddictionaries.com/us/definition/american_english/youth
• http://wiki.answers.com/Q/Is_the_word_The_youth_singular_or_plural