Age Calculator App | HTML/CSS + JAVASCRIPT + techniques ;)

Hello, super smart people! 🖐🏻

This is my first JavaScript challenge, so disappointments are guaranteed. ;D

It was a difficult challenge, especially dealing with Dates, truly a nightmare. I managed to complete the challenge the best I could. I would love to know some specifics and how you approach it.

1. How much time(days/weeks) would you take to complete this project?
2. What is the first thing you focus on? (HTML/CSS or JavaScript)?
3. How much time do you spend on research? (Googling techniques or code solutions)
4. What method/s do you take when stuck on a project?
5. How to know when "is it good enough"?

and finally, any feedback will be greatly appreciated. (And rewarded with points ;D )