Age calculator using HTML,CSS Grid AND Flex and JS

What I found difficult in the project was evaluating the date as a whole, though I managed to handle it, for the leap years, it seems to still not work. Also with my CSS styling I found it had choosing which of the method to use as I wanted to keep my code short and precise, but then I just told myself I am learning and that is why I am a Junior, to learn so I stopped stressing here I am.

I am unsure of the days calculations as they exceed 31 days, but I think it does make sense if one looks it from my point of view. Which is in the fact that I was calculating the months that one has completed in real calendar terms. Then for the days I calculated the number of days after the birthday to the end of the month and then calculated the the number of days of the new uncompleted month and then add them together.

But I am willing to learn other methods to improve on my code on how I can approach the Javascript any sites to learn javascript and understand the concepts