Try it with
XmlNode.SelectSingleNode Method :
XmlDocument docXml = new XmlDocument();
docXml.Load(diffXml);
var nsmgr = new XmlNamespaceManager(docXml.NameTable);
nsmgr.AddNamespace("xd", "http://schemas.microsoft.com/xmltools/2002/xmldiff");
XmlElement root = docXml.DocumentElement;
XmlNode node = root.SelectSingleNode("Age", nsmgr);
Console.WriteLine(node.InnerXml);
XmlNode node = root.SelectSingleNode("Phone", nsmgr);
Console.WriteLine(node.InnerXml);