ADT (using a header file, source file and main file) in C - c

Question

I am trying to create a simple ADT using a structure that takes 2 dates. Then returns an age. It must use a Header file, a source file for the Header file, and a main file.
This is what I have it runs and nothing happens. Can someone tell me what i am doing wrong?
age.h
#ifndef AGE_H_
#define AGE_H_
typedef struct getage * Age;
#define MAX 5
Age get_Age(int birthYear, int yearNow);
void age_Destroy(Age a);
#endif
age.c
#include <stdio.h>
#include "age.h"
struct getage {
int birthYear;
int yearNow;
};
Age a[1];
Age get_Age(int birthYear, int yearNow){
int giveAge = 0;
giveAge = a[0]->yearNow - a[0]->birthYear;
printf("%d",giveAge);
return 0;
}
void age_Destroy(Age a){
free(a);
}
main.c
#include <windows.h>
#include <stdio.h>
#include "age.h"
void age_print(Age a);
void age_print(Age a){
printf("%d\n", &a);
}
int main() {
Age a;
get_Age(1986, 2020);
age_print(a);
printf("%d\n", &a);
system("pause");
//age_Destroy(a);
}

Answer 1

What are wrong:
In the function get_Age:
Instead of allocating structures, a[0] (global variable, initialized to NULL) is dereferenced.
0 (converted to NULL) is returned instead of returning an age.
In the function age_Destroy:
free() is used without declaration nor including proper header.
In the function age_print:
Data having wrong type is passed to printf(): %d requests int but Age* is passed.
In the function main:
The return value of get_Age is dropped.
Data having wrong type is passed to printf(): %d requests int but Age* is passed.
Fixed code that won't cause Segmentation Fault nor undefined behavior:
age.h (not changed)
#ifndef AGE_H_
#define AGE_H_
typedef struct getage * Age;
#define MAX 5
Age get_Age(int birthYear, int yearNow);
void age_Destroy(Age a);
#endif
age.c
#include <stdio.h>
#include <stdlib.h> // for malloc() and free()
#include "age.h"
struct getage {
int birthYear;
int yearNow;
};
Age get_Age(int birthYear, int yearNow){
Age a = malloc(sizeof(*a)); // allocate a structure
if (a == NULL) { perror("malloc"); exit(1); }
a->yearNow = yearNow; // assign data
a->birthYear = birthYear;
int giveAge = 0;
giveAge = a->yearNow - a->birthYear;
printf("%d",giveAge);
return a; // return pointer to the allocated structure
}
void age_Destroy(Age a){
free(a);
}
main.c
#include <stdlib.h> // more portable header for system()
#include <stdio.h>
#include "age.h"
void age_print(Age a);
void age_print(Age a){
printf("%p\n", (void*)a); // use valid combination of format and data
}
int main() {
Age a;
a = get_Age(1986, 2020); // assign the return value
age_print(a);
printf("%p\n", (void*)a); // use valid combination of format and data
system("pause");
age_Destroy(a); // enable freeing
}
(Some behavior may look weird, but I believe this is valid because not desired behavior is described.)