Online and last seen tell you if your contacts are online, or the last time they were using WhatsApp.
Through our privacy settings, you have the option to control who can see your last seen. Please note you cannot hide your online status. Learn more about privacy settings on: Android | iPhone | Windows Phone | Nokia S40 | BlackBerry | Nokia S60 | BlackBerry 10
There are a few reasons why you may not be able to see a contact's last seen:
Cheers,
WhatsApp Support Team