validation date of birth composed of 3 dropdowns: month, day, year

I have form that contain: date of birth
date of birth is composed of 3 dropdowns:  month, day, year
I want validate the birthday to check if the user has a least 18 year old, and its valid date via custom rule like below, but it does not work,
Maybe I'm wrong, How to do something like that? Thanks

Code:

        $rules = [
            'first_name' => 'required|min_length[2]|max_length[20]',
            'last_name' => 'required|min_length[2]|max_length[20]',

            'month' => 'required',
            'day' => 'required',
            'year' => 'required',
            'birthday' => 'birthdayValidation[month,day,year]',
           
            'gender' => 'required|in_list[1,2]',
        ];

I found solution by creating the custom rule below, and it seems working great, if anyone interested

PHP Code:

<?php
namespace App\Validation;

class CustomRules{
    
    /**
    * Rule is to validate birthday
    * @param string $str
    * @param string $fields
    * @param array $data
    * @param string $error
    * @return boolean
    */
    public function birthdayValidation(string $str = null, string $fields = null, array $data = [], string &$error = null) {
    
        if(empty($data['month']) || empty($data['day']) || empty($data['year'])) {
            return true;
        }
        
        $birthDay = $data['year'].'-'.$data['month'].'-'.$data['day'];
        
        if(!checkdate($data['month'], $data['day'], $data['year'])) {
            $error = "The Birthday field is not a valid date.";
            return false;
        }
        
        if (time() < strtotime('+18 years', strtotime($birthDay))) {
            $error = "You must be at least 18 years old.";
            return false;
        }
        
        if (time() > strtotime('+99 years', strtotime($birthDay))) {
            $error = "The age may not be greater than 99 years.";
            return false;
        }
        
        return true;
    }
    
} 


Thank you for posting your solution.
But what if my 102 years old grandpa wants to register on your website? 

CodeIgniter 4 tutorials (EN/FR) - https://includebeer.com
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Thank you for posting your solution.
But what if my 102 years old grandpa wants to register on your website? 

You welcome, I thought maybe 99 is a max, I think fb do the same, not sure