Rearrange Date input field rendering
You need to change #type value to date_select

So your code should look like this 

$form['dob'] = array(
    '#type' =&gt; 'date_select',
    '#date_format' =&gt; 'm-d-Y',
    '#title' =&gt; 'Date of Birth',
    '#required' =&gt; TRUE,
);


If you want text field instead of select dropdown menu you can change date_select to date_text