Controller.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Session;
use Illuminate\Foundation\Bus\DispatchesJobs;
use Illuminate\Routing\Controller as BaseController;
use Illuminate\Foundation\Validation\ValidatesRequests;
use Illuminate\Foundation\Auth\Access\AuthorizesRequests;

class Controller extends BaseController
{
    use AuthorizesRequests, DispatchesJobs, ValidatesRequests;
    protected $thisYear;

    public function __construct(){
        $this->thisYear = $this->getYear();
    }
    
    function  submitForm(Request $request){
        $year = $request->year;
        echo $year;

        if($year >= $this->thisYear || $year <= 1900 ){
            Session::flash('error','Invalid year, year must be less than 2020 or greater than 1900');

            return redirect()->back();
        }else{
            $age = $this->thisYear - $year;
            
            Session::flash('success','You\'re ' .$age. ' years old');

            return redirect()->back();
        }
    }

    protected function getYear(){
        return (int)Date('Y');
    }


}