Calculate age from birthdate 
<p>I have two textboxes. The first one is a date, the second one is an age. I have 2 cases :</p>

<ul>
<li>I know the birthdate, I fill in the textbox and the age is calculated and displayed in the second textbox, this textbox is readonly</li>
<li>I don't know the birthdate, I enter an age, no birthdate and no more action</li>
</ul>

<p>Do you have an idea for the first point ?</p>
