For someone usually well informed your ignorance in this post baffles me Marianne.
First Willem-Alexander became the Prince of Orange on april 30th 1980 and his mother never elevated him towards that title.
Beatrix and Claus wanted to alter the age that an heir(ess) become old enough to ascend the throne to 21 or older. That would also have altered the age for the apanage to start. The Dutch parliament however had just voted that youngsters become adults at 18 years old and did not support such plans. (that would have required them to look a little further than the 2-5 years ahead they usually stick to)
Princess Margriet does not receive a complete apanage but she does declare all her functional costs and they are paid for by the Dutch state.
Meaning she does not have the personal salary part of the incomes of the monarch, consort, former monarch and heir(ess) to the throne but she is not paying for everything herself.
You also forget the scenario where the heiress from the age of 18 automatically becomes the monarch when something happens to end her father's reign or the regent when her father is unable to reign himself.
Meaning she becomes the acting understudy of the monarch upon her 18th birthday. Hopefully we will not need her to step into a formal role at that age but because she the official understudy we do need to pay for her as well. In republics with a vice-president you do pay for that post also.
Beatrix and Claus kept their sons from official roles as children only appearing occasionally at events before they become 18. However WA did do more from 18 onwards then before that age or his brothers did. Amalia will follow that same route as has been explained by her parents.
For over a century the heir to the Dutch throne would indeed be granted an annual income by the State when reaching the age of 18. The background for this was that in those days an heir(ess) ought to be fully prepared to become the Monarch upon reaching majority.
Former Queen Wilhelmina is the perfect example here, when she at the age of 10 lost her father, but she only became the rightful Queen of the Netherlands upon her Inauguarion in September 1898 after her mother, Queen Emma, had been Regent for those 8 years until Wilhelmina became 18.
However, in these days and age a royal heir is allowed far more educational years and thus 18 has become less significant.
When the present King, Willem-Alexander, became 18 years 'old' his mother Queen Beatrix granted him the title of Prince of Orange, much to the surprise of both parliament & public.
There were also fierce protests to WA's income as per that date as there was no necessity to 'set upo his own House etc.' as he was still living with his parents & siblings and starting an education at Uni & a military career (Marine a.o.).
The income Catharina-Amalia will receive on her 18th birthday may be something she would have needed had she lived in another cenury but she still lives at home with her parents and sisters, her Uni will be paid for her and she too will be allowed to study & travel and broader her horizon over many years to come, and there were plenty protest in Parliament and on the street as to the excessive height of her income, which, after all is paid for by the tax-payers.
The law that was created in a time with different age expectations ought to have been changed a long time ago. It is simply out dated!
Raising the age of receiving the benefits of such a royal allowance to 21 - even 25 years - would be closer to present days' career expectations.
We all know that Princess Margriet doesn't receive an apanage even though she, for many decades, has had a full calendar when representing her sister Queen Beatrix and even nowadays, although on a lesser scale, her nephew WA, but the Princess only gets reimbursed for her work and gets to use royal transport.
A simular arrangement might seem more appropriate for young Princess Catharina-Amalia if and when she starts representing on her own and on behalf of her father the King, until she reaches full majority!