Because these devices are 32-bit architecture (Apple’s A6 chip), modern apps—including recent versions of WhatsApp—are compiled for 64-bit systems (iOS 11 and above). This is the root of the compatibility problem.
Unless Apple releases an unexpected iOS 10.3.5 (impossible, as the iPhone 5 is marked obsolete globally), WhatsApp on this platform is . whatsapp ios 10.3.4 download