The headline above (Straits Times Online, 13 September 2010) contains a subject–verb agreement error; make it S’pore young worry MM.
The relevant sense of young here is ‘young people considered as a group’, hence it is equivalent to (one of the senses of) youth and is a collective noun.
How could we find out the grammatical properties of a noun, or indeed of any other grammatical category? Here’s where a good learner’s dictionary comes in handy. The Oxford Advanced Learner’s Dictionary, for example, marks young as ‘[pl.]’, meaning that it is grammatically plural; accordingly, it should be followed by a plural verb.