Despite being legal adults, 18-year-old workers receive only 70 per cent of the wage of a 21-year-old.
Australian Unions are backing a bid to end junior rates of pay in a case that began in the Fair Work Commission on Monday.
The retail workers union, the Shop, Distributive and Allied Employees' Association, applied to the commission to scrap junior pay rates for workers aged 18 and over in retail, fast food and pharmacies so they can be paid adult wages.
Workers aged under 21 are not currently entitled to adult wages.
They are instead paid lower wages on a sliding scale the SDA says is out of touch with cost-of-living pressures.
Workers aged 20 are paid 90 per cent of the adult award rate, a 19-year-old gets 80 per cent of the award rate and 18-year-olds are paid 70 per cent of full award rates in retail, fast food and pharmacies.
In opening statements the the full bench of the commission, union lawyers argued junior rates were "irrational" and "unfair", citing expert evidence from Professor Martin O'Brien and Professor Jeff Borland, who both estimated minimal impacts on employers' costs and total employment.
The SDA will call 60 witnesses, including people paid junior wages, but only four experts will be subject to cross examination by employer groups, including the Pharmacy Guild of Australia, over the three-week hearing.
Outside the commission hearings in Melbourne, ACTU president Michele O'Neil said an 18-year-old would need to work 50 hours to earn what an adult makes in 38.
"Once you're legally an adult, you should be paid the adult wage. It's straightforward and fair," Ms O'Neil told reporters on Monday.
"Employers argue adult pay for 18-year-olds will kill jobs. We've heard it before. They said equal pay for women would kill jobs. It didn't."
Just a month after he turned 18, Baylin Lipkevius Thompson, now aged 21, moved out of home and was paying rent and receiving no extra financial support.
He relied on his job in a local supermarket to help cover those bills, and needed to get a second job at another supermarket chain to make ends meet.
"I was in a situation where on rent day, I'm putting all my money into rent and I'm left with 10 to 15 dollars in the bank account," Baylin told reporters.
"I was struggling to fund how I was going to feed myself, and how I could cover other bills and having that second job was just a requirement.
"Junior rates are simply not enough and that's what the SDA's push for adult wage is about. It's about fairness."
If approved, Australia will join the likes of New Zealand, where young workers move to full pay after six months, and Canada, where youth rates don't exist.
Federal Industrial Relations Minister Amanda Rishworth has been contacted for comment.
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