Well its very tricky and i think very essential for peoples like us,who are keep online 24×7 on Whatsapp.So in this situation its important that you must disable the last seen option for many reasons.Lets check how you disable last seen on Whatsapp Disable Last Seen Option In Android
1.Its bit easy,Just disable mobile data option
2.Now read your messages and close the app
3.Activate mobile data connection again.Thats it Disable Last Seen Option In iPhone
It is also easy as like android.If you have latest version of whatsapp for your then it must have Iphone has Last Seen Timestamp option.
It will help to hide last seen status.Just activate through
Settings > Advanced >Last Seen Timesatmp