Linq Group by

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

public class Program
{
  public static void Main()
  {
    var list = new List<Emp>
    {
      new Emp{ Age = 1, Comp="11", Name="111"},
      new Emp{ Age = 2, Comp="11", Name="222"},
      new Emp{ Age = 3, Comp="11", Name="333"},
      new Emp{ Age = 4, Comp="22", Name="444"},
      new Emp{ Age = 5, Comp="22", Name="555"},
      new Emp{ Age = 6, Comp="22", Name="666"},
      new Emp{ Age = 7, Comp="22", Name="777"},
    };
    // avg count oldest
    var eg = list.GroupBy(e => e.Comp);
    foreach(var e in eg.ToDictionary(k=>k.Key, v=> v.Count())){
      Console.WriteLine($"{e.Key} - {e.Value}");
    }
    Console.WriteLine("***********************************");
    foreach(var e in eg.ToDictionary(k=>k.Key, v=> (int)v.Average(e=>e.Age))){
      Console.WriteLine($"{e.Key} - {e.Value}");
    }
    Console.WriteLine("***********************************");
    foreach(var e in eg.ToDictionary(k=>k.Key, v=> v.Max(e=>e.Age))){
      Console.WriteLine($"{e.Key} - {e.Value}");
    }
    Console.WriteLine("***********************************");
    foreach(var e in eg){
      var max = e.Max(f=>f.Age);
      Console.WriteLine($"{e.Key} - {e.FirstOrDefault(s=> s.Age == max)}");
    }
  }

  public  class Emp
  {
    public int Age{get;set;}
    public string Name{get;set;}
    public string Comp{get;set;}
  }
}