Add record in listGrid using variable

I am adding record in grid using startEditingNew method as below.

var COLUMN_NAME = {

name : "user_name",
lastname : "user_surname",
age : "user_age"
};

---------------------------------------

addDataToGrid : function (name, lastname, age){

MyGrid_Grid.startEditingNew({

COLUMN_NAME.name: name,
COLUMN_NAME.lastname: lastname,
COLUMN_NAME.age: age
});
}

But, my above function raise error and does not add record to grid. If I use "user_name" string instead of "COLUMN_NAME.name" , It works fine.

How can I use variable as column name??

Thanks in advance