using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string xml = @"<employees>
  <employee1 EmpID=""1"" EmpName=""洪石丹"" EmpAge=""25"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""6"" EmpName=""黄鑫"" EmpAge=""21"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""2"" EmpName=""覃园"" EmpAge=""22"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""27"" EmpName=""袁岩岩"" EmpAge=""25"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""5"" EmpName=""江冬梅"" EmpAge=""22"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""22"" EmpName=""瞿路"" EmpAge=""23"" EmpSex=""女"">
  </employee1>
  <employee1 EmpID=""42"" EmpName=""陈建勤"" EmpAge=""24"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""44"" EmpName=""陈"" EmpAge=""24"" EmpSex=""男"">
  </employee1>
  <employee1 EmpID=""45"" EmpName=""aa"" EmpAge=""20"" EmpSex=""女"" />
</employees>";
            var query = XElement.Parse(xml).Elements("employee1").Select(x => Convert.ToInt32(x.Attribute("EmpAge").Value)).Max();
            Console.WriteLine(query);
        }
    }
}

------解决方案--------------------
顺便说下，如果你查询这么写：

C# code
var query = XElement.Parse(xml).Elements("employee1").OrderByDescending(x => Convert.ToInt32(x.Attribute("EmpAge").Value)).First();