WhatsApp for Android:

First launch Whatsapp and then click on Menu in it. Commonly it's a 3-dots symbol on the top right-corner and for those who have a menu button in the three capacitive button at the bottom can access it from there (usually bottom left-most or right-most touch capacitive button).
Under the Menu user will find Settings.
Tap on Settings and find Privacy option and select.
Under the Privacy, users can see the Last Seen option under Who Can See My Personal Info. Just tap on it and select whom do you want to show the information - Everyone (info will be visible to all), My contacts (Will not be visible to people other than WhatsApp contacts) and Nobody (No one can see the info and neither can you see their Last Seen info).

WhatsApp for iPhone:

First launch Whatsapp and then tap on Settings in it, located at the bottom right corner.
Now in Settings find an option says Account.
Tap on Account and then tap on Privacy in it.
Now finally tap on Last seen to change it from Everyone (info will be visible to all) to My contacts (Will not be visible to people other than WhatsApp contacts) or Nobody

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