Responsive age calculator built with HTML, CSS and Javascript

While building this application I used a lot of if-else for validation. Also, the code that I wrote in general is pretty dirty in my opinion. If there is any way to improve the code quality or any way to modularise this code then I will be grateful if you can provide me with some suggestions as I am still learning about modular javascript. Also, if there is a simple way to write the validation checks then I will be grateful if you can provide me with some insights on the same.