Age.js

/***************************************************************************************
 * The function below sets an arrayformula in F2 of 'Student Database' that fills down *
 * column F.                                                                           *
 *                                                                                     *
 * The formula calculates the student's age based on the date of birth in column E.    *
 *                                                                                     *
 * A trigger is set to run this function on edit.                                      *
***************************************************************************************/

function insertFormula() {
  let sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Student Database");
  let targetCell = 'F2';
  let formula = '=arrayformula(DATEDIF(E2:E,TODAY(),"Y"))';

  let cell = sheet.getRange(targetCell);
  let currentValue = cell.getValue();

  if (isEmptyValue(currentValue)) {
    cell.setFormula(formula);
  } else {
    Logger.log("Good");
  }
}

function isEmptyValue(value) {
  return value === "" || value === null;
}