DSXchange

My bad , I read incorrectly.
I thought the grouping key is DUMMY column.
But now I have checked its the AgeGroup column itself.
But I don't understand that why the need for extra column, for counting purpose I can use AgeGroup column too, can't I ?

You can, but there's no simple COUNT when calculating. You can count missing values, you can count non-missing values, but there's no actual count. If you know there are no missing values then a count of non-missing values would probably suffice.