using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public class Person

    {

        public int Age;

        public string Name;

        public override string ToString()

        {

            return "Person: " + Name + " " + Age;

        }

    }

    public static void Main()

    {

        List<Person> result = new List<Person>{new Person{Age=18, Name="Hans"}};

        List<Person> result2 = new List<Person>{new Person {Age=21, Name="Christian"}, new Person {Age=21, Name="Andersen"}};

        result.AddRange( result2.Where(x => !result.Select(y => y.Age).Contains(x.Age)) );

        Console.WriteLine(string.Join("\n", result));

    }

}