How to create Custom Exception in C# ?

The code snippet is given as below.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CustomExceptionInCsharp
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                int res = checkAGE();
                Console.WriteLine(res);
                Console.ReadLine();
            }
            catch (CustomEx_AGE ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
        static int checkAGE()
        {
            Console.WriteLine("Enter AGE:");
            int age = Convert.ToInt32(Console.ReadLine());
            if (age<18 18="" :="" above="" age="" base="" be="" class="" customex_age="" exception="" msg="" new="" pre="" public="" return="" should="" string="" throw="">