Fair pay for young workers

Young workers from the age of 16-18 are paid much less than a person over the age of 18 for doing exactly the same job, yet the government still expects people aged 16-18 to pay the same price as people receiving a higher wage for things like public transport. In what way is that fair?
The minimum wage for a worker aged 16-18 is £3.87 an hour, yet the minimum wage for someone over 21 is £6.70, just because a person is younger, why should they be paid less?