(AFA - 2007)Read the paragraph and answer question

(AFA - 2007)

Read the paragraph and answer questions 26 to 28 according to it.

 

The concept of generation gap is widely accepted in Canada and the United States. It was a concept that grew in prominence in the 1960s and 1970s, when a commom belief among young people was "Never trust anyone over thirty" and many older people felt they could no longer understand young people. These days, many people think that the generation gap is lessening because the baby-boom generation (those born between 1946 and 1961) has moved past the age of thirty.

(Taken from passages 1 - TM, CUP)

 

The sentence in italic, "when a commom ... thirty" has the function of: