How are countries classified as 'ageing,' 'aged' and 'super-aged' society?

As per the United Nations, countries with more than 7% of the population aged 65 or older are classified as "ageing society," while those with over 14% are classified as "aged society". Countries with more than 20% of the population aged 65 or older are classified as "super-aged" society. South Korea has now become a "super-aged" society.