You can create custom validation rule. You can do that within service provider, e.g. in AppServiceProvider :

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use Carbon\Carbon;
use Validator;

class AppServiceProvider extends ServiceProvider
{

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Validator::extend('age_match', function($attribute, $value, $parameters, $validator) {
            $camper = \App\Models\Camper::find($parameters[0]);
            $camp   = \App\Models\Camp::find($parameters[1]);

            $now = Carbon::now();
            $age = $now->diffInYears($camper->birthdate);

            return $age >= $camp->min_age && $age <= $camp->max_age;
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {        
    }
}

Latter, validation is something like:

....
$a = $request->all();
$validator = Validator::make($a, [
        'camp_id' => 'required|numeric',
        'camper_id' => 'required|numeric|age_match:' .$a['camper_id'] . ',' . $a['camp_id'],
]);
...