I'm not sure how things work in other countries, but for Australia the National Minimum Wage is $20.33 per hour for adult rates and junior rates (under 21) are a percentage of that depending on age and Industry Award.
Generally:
• <16yo: 40% of the adult rate
• 16yo: 50% of the adult rate
• 17yo: 60% of the adult rate
• 18yo: 70% of the adult rate
• 19yo: 80% of the adult rate
• 20yo: 90% of the adult rate
A lot of retail and hospitality businesses advertise junior positions mostly looking for 15-17 year olds, and it's safe to assume it's so they have lower labour costs.
How do you think minors in particular should be paid?