Yosef Karo

“ And this should be given on the day before Purim prior to praying the Mincha prayer (Maharil) , and such is the custom in all these provinces. And one should give three half-coins of the bigger [coins] in these provinces, for there is no coin that has a half beside for [the bigger coin] , and in Austria, they give three half-Vienners, which are also called "halves." Similarly should be done in each province. And only someone who is twenty or older is obligated to give. ”