I appreciate both Barbara and Howard catching my error and responding with an answer with sources!
I think you are mistaken when you said "The Dutch age of majority was 21...".
I think Barbara was correct that it was 25 based on Roman-Dutch law. I do not have a primary source for this, but instead I refer you to this article:
"The Paternity of Aert Theunissen Middagh: A suggestion;
The Ancestry of Theunis Gysbertszen Bogaert: A Caveat"
by Cameron Allen in TAG vol. 36, no. 3 (July 1960), pp 129 - 136. (TAG up through vol. 48 is now available to members online at the NEHGS(now American Ancestors) website.)
Among other things, the above article also deals with the situation in which the date at which a young man acquired property was known, and they wanted to determine what that would say about his age.
It turns out that, "Roman-Dutch Law permitted a child of whatever age to acquire land." (p. 133) He cites for this: Robert Warden Lee,An Introduction to Roman-Dutch Law, 1953, 5th ed., p. 47.
The article also on p. 133 mentions that the age of majority was 25, citing Lee, p. 43.