If you delete that WhatsApp text before 7 minutes and used “delete for everyone” option, Then other person will know that you deleted the text. If it is a old text, then other person wont know that you deleted WhatsApp text.

But you should remember that, deleting a old WhatsApp text, wont delete it from other person’s conversation. They can still see the message. So if you delete a immediate text, other person will know, that you had deleted. If it is a old text, other person wont know.