您可以检查

float version = [[[UIDevice currentDevice] systemVersion] floatValue];

，然后显示UIAlertView.但是该应用程序甚至无法启动，因为您在项目设置中设置了操作系统要求.

and then display a UIAlertView. But the App won't even start, because OS requirement you set in your project settings.

但是请记住:用户仍然可以下载.

But remember: The User will be still able to download it.