NO, they won’t go to Hell over that one belief, provided that they have already accepted the Lord Jesus Christ as their Savior, repented from their sins & try to follow His Word afterwards.
I don’t see why the earth can’t be 6 billion yrs old, or whatever the scientists believe that it may be. The verse you gave (2 Peter 3:8) might give an inkling about that, cuz none of us really knows — where does the Bible say that the day in the Genesis creation was 24 hrs long? There was a period of time betw the creation of earth & the rime that God created Man & breathed the breath of life into him. God works on HIS time, not ours.
Amen!