Fairness is an elusive goal. People who work half time (perhaps because it is their second job) might find it unfair that the disabled person receives twice as much pay for the same amount of work.

Young people are also exempted from the minimum wage. As inexperienced workers, they can't produce the same value for an employer as older, more experienced workers.

In 2013, New Zealand recognized that minimum wage was locking young, inexperienced people out of the job market, and therefore implemented a "starting-out wage" of 80% of the adult minimum wage, saying

The starting out wage is part of a range of measures that support more young people entering the workforce (and gaining the education and training they need for work). It will influence the demand for young people by employers, and therefore encourage them to create more jobs.

For young people who can produce at least 80% of the value of adult minimum wage workers, the law no longer prevents them from entering the job market and gaining experience.

Since 2013, the numbers are improving, but over 20% of young people seeking work remain unemployed.

Working at a low-paying job ("slave wage" is meaningless) is worse than what many people do for income. But it may be better than any alternative that some people have.

You do society no favors by banning others from voluntarily acting on opportunities simply because you would refuse them.