Online and last seen status tell you if your contacts are online, or the last time they were using WhatsApp.
Online status means that contact has WhatsApp open in the foreground on their device and is connected to the internet. However, it does not necessarily mean they have read your message.
Last seen status refers to the last time the contact used WhatsApp. Through our privacy settings, you have the option to control who can see your last seen status. 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 status:
WhatsApp Support Team