Link: background info (in Dutch)
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!
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