How can I invoke the functions in the main() function?

How can I invoke the functions in the main() function? My question is how to fix the following code ? Because I can not enter any input.

int
main()
{
    int a;
    printf ("Enter your age : ");
    return 0;
}



// implementation of function 
int age()
{
    int a;

    printf("Enter a age:");
    scanf("%d",&a);
    printf("%d",a);

    return 0;
}