需要帮助来完成这个

请帮助我，我需要完成这个程序工作，但它不会按年龄排序，它应该根据年龄排序的记录，请帮助。我不能让它工作

这是我目前所做的。我不知道出了什么问题

#include <cstdlib>
#include <iostream>
using namespace std;
struct contact
{
    char lastname[30];
    char firstname[30];
    int age;
    int cnumber;
}
c[20];
void quickSort(int arr[], int left, int right) {
    int i = left, j = right;
    int tmp;
    int pivot = arr[(left + right) / 2];
    /* partition */
    while (i <= j) {
        while (arr[i] < pivot)
            i++;
        while (arr[j] > pivot)
            j--;
        if (i <= j) {
            tmp = arr[i];
            arr[i] = arr[j];
            arr[j] = tmp;
            i++;
            j--;
        }
    };
    /* recursion */
    if (left < j)
        quickSort(arr, left, j);
    if (i < right)
        quickSort(arr, i, right);
}
void addContact(int ctr)
{
    cout<<"ADD CONTACT"<<endl;
    cout<<"LAST NAME: "<<endl;
    cin>>c[ctr].lastname;
    cout<<"FIRST NAME: "<<endl;
    cin>>c[ctr].firstname;
    cout<<"AGE: "<<endl;
    cin>>c[ctr].age;
    while (c[ctr].age >= 100 || c[ctr].age <= 0)
    {
        cout<<"Input Age again: ";
        cin>>c[ctr].age;
    }
    cout<<"CONTACT NUMBER: "<<endl;
    cin>>c[ctr].cnumber;
    while (c[ctr].cnumber > 9999999 || c[ctr].cnumber < 1000000)
    {
        cout<<"Input Number Again: ";
        cin>>c[ctr].cnumber;
    }
    system("cls");
}
void display(int a)
{ cout<<"RECORDS"<<endl;
    for(int i=0;i<a;i++)
    {
        cout<<"n";
        cout<<"LAST NAME: ";
        cout<<c[i].lastname<<endl;
        cout<<"FIRST NAME: ";
        cout<<c[i].firstname<<endl;
        cout<<"AGE: ";
        cout<<c[i].age<<endl;
        cout<<"CONTACT NUMBER: ";
        cout<<c[i].cnumber<<endl;
    }
}
int main(int argc, char *argv[])
{
    int choice, loop=0, tmp;
    while (choice!=4)
    {
        cout<<"n";
        cout<<"CHOOSE"<<endl;
        cout<<"1.Add contacts: "<<endl;
        cout<<"2.Display "<<endl;
        cin>>choice;
        switch(choice)
        { case 1:
                addContact(loop);
                loop++;
                break;
            case 2:
                system("cls");
                display(loop);
                break;
            case 3:
                quickSort(&c[loop].age,loop,0);
                display(loop);
                break;
            default:
                cout<<"Invalid";
        }
    }

    return 0;
}