I see your point. It is age discrimination and seems unfair.
Trying to think of the reason it is done like this - Perhaps they think that, generally, 16-17 yr olds are likely to be living at home and have few expenses, and 18-20yr olds the same, with perhaps a few more expenses, but neither group - for the most part - is likely to be living independently?
Or course, there are those who are. But I expect that the vast majority are still at home.
so they don't have the same outgoings.
Then - how to get employers to choose to take on a young person with less experience over an older person?
I'm sure everyone would love to think employers don't think like this
but you'd be wrong. Cost matters.
If you had to pay the same, what would motivate you to take on a 16, 17, 18 etc yr old rather than a 25 or 30yr old? What impact would that have on youth unemployment?
Just trying to get a feel for it and think of possible reasons why they've set it up this way.
If minimum wage was standard - why would employers take on a 16yr old and train them, when they'd have to pay the same as for a 30yr old who's done the job before?
I'm not saying this is morally right or anything
but I suspect it's probably at least part of the picture.