My grandmother was born in 1914 and died in 1999 and never left australia.
Assuming she was a precocious child and designed a boat before her 10th birthday (or wrote a book or painted a picture etc.) such that it was created before 1924. Copyright in it will last until 31 December 2069, 70 years after her death, as it will for all the copyrighted works she created, and her heirs will be able to exercise them.
If she had travelled to mexico then copyright on the works she created while there would last until 2099, 100 years after her death.
Of course, the United States is more complicated. If the boat design had been created there and published and registered before 1978, copyright would have expired. However, if it wasn’t published until after 1978, then it is copyright until 2047. Of course, my grandmother’s boat design was only published in australia before 1992 and since the US doesn’t use the rule of the shorter term it enjoys copyright until 2069.
Please make your royalty cheque payable to the undersigned.