2employeesManagement.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Solution
{
    public class Solution
    {
        public static Dictionary<string, int> AverageAgeForEachCompany(List<Employee> employees){
            var result = new Dictionary<string, int>();
            foreach (var company in employees.Select(x => x.Company).Distinct().OrderBy(x => x))
            {
                result.Add(company, (int)Math.Round(employees.Where(x => x.Company == company).Average(y => y.Age), 0));
            }

            return result;
        }

        public static Dictionary<string, int> CountOfEmployeesForEachCompany(List<Employee> employees){
            var result = new Dictionary<string, int>();
            foreach (var company in employees.Select(x => x.Company).Distinct().OrderBy(x => x)){
                result.Add(company, (int)employees.Where(x => x.Company == company).Count());
            }
            return result;
        }

        public static Dictionary<string, Employee> OldestAgeForEachCompany(List<Employee> employees){
            var result = new Dictionary<string, Employee>();
            foreach (var company in employees.Select(x => x.Company).Distinct().OrderBy(x => x)){
                result.Add(company, employees.Where(x => x.Company == company).OrderByDescending(y => y.Age).First());
            }
            return result;
        }

        static void Main(){
            int countOfEmployees = int.Parse(Console.ReadLine());
            var employees = new List<Employee>();

            for (int i = 0; i < countOfEmployees; i++){
                string str = Console.ReadLine();
                string[] strArr = str.Split(' ');
                employees.Add(new Employee {FirstName = strArr[0],
                                            LastName = strArr[1],
                                            Company = strArr[2],
                                            Age = int.Parse(strArr[3])});
            }

            foreach (var emp in AverageAgeForEachCompany(employees)){
                Console.WriteLine($"The average age for company {emp.Key} is {emp.Value}");
            }

            foreach (var emp in CountOfEmployeesForEachCompany(employees)){
                Console.WriteLine($"The count of employees for company {emp.Key} is {emp.Value}");
            }

            foreach (var emp in OldestAgeForEachCompany(employees)){
                Console.WriteLine($"The oldest employee of company {emp.Key} is {emp.Value.FirstName} {emp.Value.LastName} having age {emp.Value.Age}");
            }
        }
    }

    public class Employee
    {
        public string FirstName { 
            get;
            set; 
        }
        public string LastName { 
            get;
            set;
        }
        public int Age {
            get;
            set; 
        }
        public string Company {
            get;
            set;
        }
    }
}