Age calculator using HTML, CSSand Javascript

This project was an oppourtunity to familiarize myself with JS. Initially it seemed like a daunting task but finally I managed to pin it down and complete it.

My only hurdle which I can't think to clear is that how do I keep the textboxes horizontally aligned when I make the error text appear. Whenever there is an error(basically incorrect input), the particular textbox under which the error text is supposed to appear seems to get lifted from its position. How do I correct this? Thankyou.