Trying to construct the date by concatenating three casts from int to nvarchar() is likely to be somewhat error-prone, and even if you get it right (eliminating spaces, ensuring month and day parts are two characters) it will perform very badly. It's easier to get right and might perform slightly better (not much - not having the the DOB as a datetime value will be a big performance drag) if you use DATEADD to construct the date -

DATEADD(day, DOB_DAY, DATEADD(month, DOB_MONTH, DATEADD(year, DOB_YEAR, 0)))

There's no need for any casting and it avoids the implicit conversion from nvarchar to datetime if you do it like that.