Happy to help. You can make it a bit neater by putting the CASE logic near the top:

...

SET @carage =CASE

-- don't need to cast as int because that's the data type of the variable

WHEN YEAR(GETDATE() - @carage) < 6 THEN 1

ELSE 2

END

...

AND COALESCE(CarAgeGroup, @carage) = CarAgeGroup

END

John