Steps to hide your  'last seen' in telegram:

• To access this option, go to the bottom of the screen or window and tap or click on the Settings cog.
• Select 'Privacy and Security' from the menu that appears.
• Under the Privacy heading, tap 'Last Seen & Online.'
• You can decide who can see your 'Last Seen Online' time on the next screen: Nobody, Everyone (including users you haven't added), and My Contacts
• You can add exceptions to this rule depending on which configuration you choose.

Read More: Why do we use Telegram?