PHP iFrame Conditional

I am trying to get it so if two conditionals match then both iframes work, but it only shows one or the other.

if ($_POST["age"] === '13below')
{
echo '<iframe id="13" width="80%" height="400px" src="https://domain.com"></iframe>';
}
if ($_POST["age"] === '18plus')
{
echo '<iframe id="18" width="80%" height="400px" src="https://domain.com"></iframe>';
}