CalculateAge.gs

const TAG_NAME = 'calculatedAge';
const TITLE_APPENDIX = "Geburtstag"
const CALENDAR_ID = "<your-calendar-id>"
const FORCE_OVERWRITE = true;

function calcAge5() {
  calculateAgeForYears(5)
}

function calculateAgeForYears(years) {
  for (var years_offset = 0; years_offset <= years; years_offset++) {
    calculateAge(years_offset);
  }
}

/*
 * Calculate the age of the birthday people in each year and write it in the description
 */
function calculateAge(years_offset) {
  // Get Calendar 'Birthdays'
  var birthdayCal = CalendarApp.getCalendarById(CALENDAR_ID);

  // Select date range of current year
  var currentYear = (new Date()).getFullYear() + years_offset;
  var start = new Date(currentYear + '-01-01');
  var end = new Date(currentYear + '-12-31');

  // Fetch events from Birthday Calendar
  var birthdays = birthdayCal.getEvents(start, end);

  // Filter Birthdays out of Default Calendar (if no specific Birthday Calendar is present)
  // birthdays = birthdays.filter(filterBirthdays);

  var calculatedAge;
  var old_title;
  var new_title;
  var name;

  for (var i = 0; i < birthdays.length; i++) {
    e = birthdays[i];

    // Year of birth is stored in the Location field of the event
    if (e.getLocation() !== "") {

      // Calculate the age if it has not been done before OR FORCE_OVERWRITE is true
      if ((e.getDescription() === "" || isNaN(parseInt(e.getTag(TAG_NAME)))) || FORCE_OVERWRITE) {

        calculatedAge = Math.round(currentYear - parseInt(e.getLocation()));
        old_title = e.getTitle()
        //name = old_title.substr(0, old_title.lastIndexOf(" "));
        name = getNameFromString(old_title)
        new_title = insertAgeIntoString(old_title, calculatedAge);

        // Customize the description here
        e.setDescription(name + ' wird heute ' + calculatedAge + ' Jahre alt!');
        e.setTitle(new_title);


        // Save calculated age in tag
        e.setTag(TAG_NAME, calculatedAge);

        Logger.log(name, ' | ', e.getTitle(), ' | ', e.getTag(TAG_NAME), ' | ', e.getDescription());

      }
    }
  }
}

/*
 * Insert the age into the title
 */
function insertAgeIntoString(originalString, age) {
  // Regular expression to find the pattern "(age)"
  const agePattern = /\(\d+\)/;

  // Find the position where title appendix starts
  const position = originalString.indexOf(TITLE_APPENDIX);

  if (position === -1) {
    // If the title appendix is not found, return the original string
    if (agePattern.test(originalString)) {
      // Replace the existing age with the new age
      return originalString.replace(agePattern, `(${age})`);
    } else {
      return originalString + ` (${age})`;
    }
  }

  // Check if there is already an age in parentheses before title appendix
  const substringBefore = originalString.slice(0, position).trim();
  if (agePattern.test(substringBefore)) {
    // Replace the existing age with the new age
    return originalString.replace(agePattern, `(${age})`);
  } else {
    // Insert the new age before the title appendix
    return substringBefore + ` (${age}) ` + originalString.slice(position);
  }
}

/*
 * Get Name from the title string
 */
function getNameFromString(originalString) {
  // Regular expression to find the pattern "(age)"
  const agePattern = /\(\d+\)/;

  // Find the position where title appendix starts
  const position = originalString.indexOf(TITLE_APPENDIX);

  if (position === -1) {
    // If the title appendix is not found, return the original string
    return originalString.replace(agePattern, '').trim();
  }

  // Get the substring before title appendix
  const substringBeforeGeburtstag = originalString.slice(0, position).trim();

  // Remove the age pattern if it exists
  return substringBeforeGeburtstag.replace(agePattern, '').trim();
}

/*
 * Filter the birthdays out of the fetched events
 * Add additional filters if you don't have a specific Birthday Calendar
 */
function filterBirthdays(event) {
  // Add filter here (e.g. event.getColor === CalendarApp.Color.YELLOW)
  return event.isAllDayEvent();
}