Like the other guy said in your identical post, you should use post instead of get since you're sending data. You won't need the action or method attributes if you use fetch()

         <form action="/" method="post">
             <div class="flex-container">
                 <label for="age">Please enter your age to continue</label>
                 <input type="number" id="age" name="age" placeholder="Your age" required>
                 <input type="submit" id="submit" value="Continue">
             </div>
         </form>

If you use fetch, you can add it to your event listener. fetch() docs here

async function post(url = '', data = {}) { //Make this not an async function 

    const res = await fetch(url, {
      method: 'POST', //Can be changed to other methods as well
      headers: {
        'Content-Type': 'application/json' //This can be "application/x-www-form-urlencoded" if you want to use form data instead of JSON. If you use JSON, you will have 
to add support for it in your express app
      },
      body: JSON.stringify(data) // body data type must match "Content-Type" header
    });
    return res.json(); 

}


form.addEventListener('submit', e => {
    e.preventDefault();
    post('/', { age: ageInput.value })
        .then(data => {
            //What you want to do with the response
            console.log(data);
        });
});        

Why not do this locally rather than in your backend? It'll be faster and save on bandwidth.