Hong Kong's population has increased steadily over the past decade, reaching about 6.8 million by 2002. Hong Kong is one of the most densely populated areas in the world, with an overall density of some 6,250 people per square kilometer. Cantonese, the official Chinese language in Hong Kong, is spoken by most of the population. English, also an official language, is widely understood. It is spoken by more than one-third of the population. Every major religion is practiced freely in Hong Kong. All children are required by law to be in full-time education between the ages of 6 and 15. Preschool education for most children begins at age 3. Primary school begins normally at the age of 6 and lasts for 6 years. At about age 12, children progress to a 3-year course of junior secondary education. Most stay on for a 2-year senior secondary course, while others join full-time vocational training. More than 90% of children complete upper secondary education or equivalent vocational education.