DataTable dt2 = dt1.Copy();
            dt2.Clear();
            int age = 0;
            int money = 0;
            for (int i = 0; i < dt1.Rows.Count; i++)
            {
                if (i < 3)
                    dt2.Rows.Add(dt1.Rows[i]);
                else {
                    age += int.Parse(dt1.Rows[i]["age"].ToString());
                    money += int.Parse(dt1.Rows[i]["money"].ToString());
                }
            }

            DataRow dr2 = dt2.NewRow();
            dr2["id"] = 4;
            dr2["name"] = "其它";
            dr2["age"] = age;
            dr2["money"] = money;

            dt2.Rows.Add(dr2);

------解决方案--------------------

C# code
DataTable dt = CreateDataTable();//得到你的datatable
                var result= dt.AsEnumerable().Where((a, index) => index >= dt.Rows.Count - 2).CopyToDataTable<DataRow>();
                dt = dt.AsEnumerable().Where((a, index) => index < dt.Rows.Count - 2).CopyToDataTable<DataRow>();
                DataRow dr = dt.NewRow();
                dr["ID"] = dt.AsEnumerable().Last().Field<int>("ID") + 1;
                dr["name"] = "其他";
                dr["age"] = result.Compute("sum(age)",null);
                dr["money"] = result.Compute("sum(money)", null); ;
                dt.Rows.Add(dr);

                //此时dt已符合要求

------解决方案--------------------
C# code
        DataTable dt = new DataTable();
        dt.Columns.Add("id", typeof(int));
        dt.Columns.Add("name", typeof(string));
        dt.Columns.Add("age", typeof(int));
        dt.Columns.Add("money", typeof(int));
        dt.Rows.Add(1, "张一", 10, 10);
        dt.Rows.Add(2, "张二", 15, 20);
        dt.Rows.Add(3, "张三", 20, 30);
        dt.Rows.Add(4, "张四", 18, 40);
        dt.Rows.Add(5, "张五", 25, 50);
        int ageSum = (int)dt.Rows[dt.Rows.Count - 2]["age"] + (int)dt.Rows[dt.Rows.Count - 1]["age"];
        int moneySum = (int)dt.Rows[dt.Rows.Count - 2]["money"] + (int)dt.Rows[dt.Rows.Count - 1]["money"];
        dt.Rows.RemoveAt(dt.Rows.Count - 1);
        dt.Rows[dt.Rows.Count - 1]["name"] = "其他";
        dt.Rows[dt.Rows.Count - 1]["age"] = ageSum;
        dt.Rows[dt.Rows.Count - 1]["money"] = moneySum;