Verify age is over 18 when using WP Job Manager Field Editor (Resume/Candidate)

functions.php

<?php
add_filter( 'submit_resume_form_validate_fields', 'smyles_verify_age_field_over_18' );

function smyles_verify_age_field_over_18( $no_errors, $fields, $values ) {

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

	if ( empty( $values['resume_fields']['candidate_age'] ) || absint( $values['resume_fields']['candidate_age'] ) < 18 ) {
		throw new Exception( __( 'You must be 18 years or older to submit a listing.' ) );
	}
	
	return $no_errors;
}

If you want to use a date picker for the "birthday" instead, use this code:
https://gist.github.com/tripflex/7a5e6e83879f0910ceaeab16b27c67a0