Hi there,
you have to put your function above the main function.
the enginge reads your script from the top till downside.
like reading a book.

...
..
.
///////////////////////////////////////////////////////////////
function compute_days()
{
number_of_days = my_age * days_a_year;
}
///////////////////////////////////////////////////////////////
function main()
{
screen_color.blue = 150;
compute_days();
}
///////////////////////////////////////////////////////////////

cheers