Responsive Age Calculator App using CSS Flexbox, having last date

I was successfully able to disable the button in case of future year, or future month in same year, or future day in same month of same year, but when it comes to detecting an incorrect month input like month > 12 or incorrect day input like day > 31, it gives the correct error message, but I am not able disable the button in the page. Can anyone who reviews my code tell me what I am doing wrong? Thank you.