This is is a very common and popular assumption about who makes up the minimum wage workforce. It is repeated so often that it is no longer even questioned by a majority of people. While it is true that a large proportion of the minimum wage workforce is made up of teenagers, it is by no means a majority, as can be seen in the Labour Statistics Research Paper published by Statistics Canada in September 2019 - Maximum insights on minimum wage workers: 20 years of data. As can be seen in the graph below, the trend is in the opposite direction, with more and more older workers joining that workforce.
The percentage of workers aged 15 to 24 years making up the minimum wage workforce was 60.7% in 1998, 63.6% in 2008, 52.3% in 2018. Even if teenagers made up as much as half of that cohort, that would still only be 32% in 2008 and declining to 26% in 2018 - not even close to "most...".
Source: Statistics Canada, Labour Force Survey
See https://www150.statcan.gc.ca/n1/pub/75-004-m/75-004-m2019003-eng.htm
