How can we see when we opened the iCloud account?

Your usage is not logged, so I have seen no place where you would see when an account was opened. You can try and contact Apple directly to see if it is information they have.

Contact Apple for support and service - Apple Support

The closest I can think of telling you how to do that is if when you created the account you also signed into the community here. Then if you go to your preferences it should show you the year you joined.