How to count the elements of an array of struct in C?

Question

I need to build a function that gets an array of struct as a parameter. The struct looks like that

struct Person {
  char gender;
  char eyecolour;
  char haircolour;
  int age;

};

The function should return the age of the oldest person in the array. So, for that, I believe I have to know how many elements are there in the array given as the parameter, then I can figure out the oldest age with a for and an if.