Why isn't it enough information for him? Because the house number he was looking at was "36". How do we know that? Because if it were any other number, it would have been enough info for him to know the daughters' ages, since every other product appears only once.
So we know the girls' ages must be either
1, 6 and 6, or
2, 2 and 9
"That's right, I have to add that my eldest daughter has a dog."
He has an "eldest daughter". That means it must be "2, 2 and 9" because if it were "1, 6 and 6", then he wouldn't have an "eldest daughter", he'd have TWO eldest daughters.