This program will read age of 15 person one by one (using while loop) and count total number of Baby Age, School attending person age and Adult age person.

#include <stdio.h>
int main()
{
	int age;
	int cnt_baby=0,cnt_school=0,cnt_adult=0;
	int count=0;
	
	while(count<15)
	{
		printf("Enter age of person [%d]: ",count+1);
		scanf("%d",&age);
		
		if(age>=0 && age<=5)
			cnt_baby++;
		else if(age>=6 && age<=17)
			cnt_school++;
		else
			cnt_adult++;
		
		//increase counter
		count++;
	}
	
	printf("Baby age: %d\n",cnt_baby);
	printf("School age: %d\n",cnt_school);
	printf("Adult age: %d\n",cnt_adult);
	
	return 0;
}

Output

Enter age of person [1]: 0 
Enter age of person [2]: 1 
Enter age of person [3]: 2 
Enter age of person [4]: 3 
Enter age of person [5]: 44
Enter age of person [6]: 55
Enter age of person [7]: 66
Enter age of person [8]: 44
Enter age of person [9]: 12
Enter age of person [10]: 13 
Enter age of person [11]: 14 
Enter age of person [12]: 55 
Enter age of person [13]: 66 
Enter age of person [14]: 18 
Enter age of person [15]: 19 
Baby age: 4
School age: 3
Adult age: 8