Show age in years calculated from a Types date field

Add the following code to register a custom shortcode ‘age’:

/**
 *  Calculate age (in years) from date field
 *  specified as shortcode attribute 'slug'
 */
add_shortcode( 'age', function( $atts = [], $content = null ){

	// provide defaults
	$atts = shortcode_atts( 
		array(
			'slug'		=>	null
		), 
		$atts 
	);

	$age = '';

	if ( isset( $atts['slug'] ) ) {

		global $post;
		$timestamp = get_post_meta( $post->ID, 'wpcf-' . $atts['slug'], true );

		$date = DateTime::createFromFormat( "U", $timestamp );

		$now = new DateTime();

		$diff = $now->diff( $date );

		$age = $diff->y;
	}

	return $age;
});

You would use it like so to output the age in years calculated from a Types date field with a slug specified by the slug shortcode attribute:

Age: [age slug='date-of-birth']