Create function in infopath form from sharepoint list value

First, you need to set an onChange event on your 2 dropdown boxes. To do so, right click your dropdown boxes --> programming --> change event. This will create a visual studio project with an empty function for your dropdown box. In this function, write this code (or something similar) :

XPathNavigator root = MainDataSource.CreateNavigator();
string FirstName = root.SelectSingleNode("<XPath of your FirstName dropdown box>", NamespaceManager).Value;
string LastName = root.SelectSingleNode("<XPath of your LastName drop down box>", NamespaceManager).Value;

SPSite oSiteCollection = newSPSite(<URL of your server, eg http://portal/>);
SPWeb oWebsiteSrc = oSiteCollection.AllWebs["<URL of the site containing the sharepoint list, eg /sites/finance/expenseaccount>"];
SPList oList = oWebsiteSrc.Lists["<Name of your list>"];

string camlQuery = "<Where><And><Contains><FieldRef Name='Name of the FirstName column'/><Value Type='Text'>" + FirstName + "</Value></Contains><Eq><FieldRef Name='Name of the LastName column' /><Value Type='Text'>" + LastName + "</Value></Eq></And></Where>";
SPQuery query = newSPQuery();
query.Query = camlQuery;

SPListItemCollection collListItemsSrc = oList.GetItems(query);

root.SelectSingleNode("<XPath of your Age Field>", NamespaceManager).SetValue(collListItemsSrc[0].GetFormattedValue("<Name of the Age column>"));