age-calculator-app-main

You don't need to listen to the same event on the same element for each function (DRY). You can call an anonymous function for all the functions you need:

button.addEventListener("click", e => {
  newYear();
  newMonths();
  newDays();
});

Hello,

A few recommendations for your JS code.

1. To insert simple text do not use innerHTML, for this task use InnerText or textContent.
2. No need to create hidden class, use [hidden] attribute.
3. DRY (Don't repeat yourself) - You have large chunks of repetitive code. Lines 21-24 and 26-29, 46-49 and 51-54, 69-73 and 75-77,79 are absolutely identical. In this case, it is necessary to create functions that will perform the same task, and pass references to input, output, error, etc. tags as arguments:

const showError = (label, input, output, msg) => {
  msg.hidden = true; // instead of: msg.classList.remove("hidden");
  label.classList.replace("text-SmokeyGrey", "text-LightRed");
  input.classList.replace("border-LightGrey", "border-LightRed");
  output.textContent = "--"; // instead of: output.innerHTML = "--";
}

// showError(yearsText, inputYear, years, errorMsg3);
// showError(monthText, inputMonth, months, errorMsg2);
// showError(dayText, inputDay, days, errorMsg1);

The same for hideError()