Please try this way

use Validator;

public function store(Request $request)
{
    //$data = $request->all();
    $data = json_decode($request->payload, true);
    $rules = [
        'name' => 'digits:8', //Must be a number and length of value is 8
        'age' => 'digits:8'
    ];

    $validator = Validator::make($data, $rules);
    if ($validator->passes()) {
        //TODO Handle your data
    } else {
        //TODO Handle your error
        dd($validator->errors()->all());
    }
}

digits:value

The field under validation must be numeric and must have an exact length of value.