You can't just compare these figures in isolation.
The last time I had a proper job, I think I earned about £9 an hour (or would do if I had set hours; working over 40 hours didn't result in any more pay).
After taking off deductions, mortage, bills, all the essentials, I might have had 50p per hour to spend for myself. If I wanted to buy something costing £10 (say, a CD; this was 20 years ago...), I'd need to work 20 hours.
Now take a kid doing a part-time job on Saturdays, earning £2 an hour cash. He need only work 5 hours to buy the same CD. Five hours of unskilled work with little responsibility, versus 20 hours of skilled work. Why should such a person need a minimum wage, which would result in his needing to work even fewer hours?