Minimum jeugdloon exists since 1974. Of course the numbers change but the ratio doesn't.
Look at it like this: if 18y minimum wage was €3 back then and would double to €6 at 21y, and you're a construction worker working for €8.50 at 18y, you're sure as shit going to demand a raise at 19, 20 and 21 because all the people making minimum wage are getting those raises too. Maybe not a doubling, but you wouldn't (shouldn't) gnash your teeth and still make €8.50 three years later.
Look at it like this: if 18y minimum wage was €3 back then and would double to €6 at 21y, and you're a construction worker working for €8.50 at 18y, you're sure as shit going to demand a raise at 19, 20 and 21 because all the people making minimum wage are getting those raises too. Maybe not a doubling, but you wouldn't (shouldn't) gnash your teeth and still make €8.50 three years later.