Age calculator for React, styled-components, Vitest practice

I probably overlooked a simpler approach, but I found calculating the age surprisingly tricky. I used it as an opportunity to familiarize myself with test-driven development (just for the logic part -- not so much on the rendered output). It was good fun, but I can't help feeling there's a better solution.

(PS I know I didn't implement the leap year edge case...hopefully there aren't any other holes in it though!)