Responsive Age Calculator with SASS and JavaScript

Hi everyone, this is my first try putting my learning into practice by building an age calculator from the ground up using JavaScript and SASS. This project is considered a bit tough for me and I do appreciate any feedback from the community regarding anything about my project. Kindly comment on:

1. html - semantic elements usage and any improvement to be made.
2. css
3. javascript - logic - I need help to verify my javascript logics and comments to make my code cleaner and better.

Thank you and have a nice day =).