Using map

function openOrSenior(data) {
  const newData = data.map((item) => {
    if (item[0] >= 55 && item[1] > 7) {
      return 'Senior'
    } else {
      return 'Open'
    }
  });
  return newData;
}

openOrSenior([
  [18, 20],
  [45, 2],
  [61, 12],
  [37, 6],
  [21, 21],
  [78, 9],
]);