Validate a Job/Listing or Resume Date Picker field is 18 years or more in the past (validate 18 years or older) when using WP Job Manager Field Editor

job.php

<?php

add_filter( 'submit_job_form_validate_fields', 'smyles_validate_job_birthday_over_18', 10, 3 );

/**
 * Validate a Job/Listing Date Field is 18 years or more in the past
 *
 *
 * @param $has_error
 * @param $fields
 * @param $values
 *
 * @return bool|\WP_Error
 * @throws \Exception
 * @since @@version
 *
 */
function smyles_validate_job_birthday_over_18( $has_error, $fields, $values ) {

	$field_meta_key = "your_birthday";

	// Return true if this field doesn't exist (to prevent errors if you dont have field created)
	if ( ! isset( $values['job'][ $field_meta_key ] ) ) {
		return true;
	}

	$entered_date = $values['job'][ $field_meta_key ];

	// Parse into date object
	$parsed_date = new DateTime( $entered_date );

	$parsed_date->add( new DateInterval( "P18Y" ) );

	// Add 18 years
	if ( $parsed_date > new DateTime() ) {
		return new WP_Error( 'validation-error', __( 'You must be 18 years or older!' ) );
	}

	return $has_error;
}

resume.php

<?php

add_filter( 'submit_resume_form_validate_fields', 'smyles_validate_resume_birthday_over_18', 10, 3 );

/**
 * Validate a Resume Date Field is 18 years or more in the past
 *
 *
 * @param $has_error
 * @param $fields
 * @param $values
 *
 * @return bool|\WP_Error
 * @throws \Exception
 * @since @@version
 *
 */
function smyles_validate_resume_birthday_over_18( $has_error, $fields, $values ) {

	$field_meta_key = "your_birthday";

	// Return true if this field doesn't exist (to prevent errors if you dont have field created)
	if ( ! isset( $values['resume_fields'][ $field_meta_key ] ) ) {
		return true;
	}

	$entered_date = $values['resume_fields'][ $field_meta_key ];
	// Parse into date object
	$parsed_date  = new DateTime( $entered_date );
	// Add 18 years
	$parsed_date->add( new DateInterval( "P18Y" ) );

	if ( $parsed_date > new DateTime() ) {
		return new WP_Error( 'validation-error', __( 'You must be 18 years or older!' ) );
	}

	return $has_error;
}

If you instead want to use an "age" field, use this code (for resumes):
https://gist.github.com/tripflex/db58fdcbb790ff4641f2b1e19c9f8883