As in your original method, you don't need to calc everyone's age, just check the correct date range.  Like this:

WHERE Birthdate >= DATEADD(DAY, 1, DATEADD(YEAR,...

No.  Actually, in RC level, the...

You can, but you shouldn't.  If you're creating only one index on a temp table, make it a clustered index not a non-clus index; the key columns don't have to...

Don't sort the query, sort the result table instead, which is more useful anyway.  Even if sort the query, you will still have to sort again when reading from the...

You should cluster the staging table on the DM_KEY_HASH also, assuming you can compute the hash key as the row is being INSERTed into the staging table.  SQL is having to...

When you create a FK constraint, you can specify whether the key can be NULL or not.  If the creating code says it can't be set to NULL -- either...

If @value is a single value -- and it seems like it would have to be only a single value -- then wouldn't it be simpler and more efficient to...

Pass the RAND value in.  And, for efficiency, get rid of the local variables in the function.

SET ANSI_NULLS ON;
SET QUOTED_IDENTIFIER ON;
GO

CREATE function...

You might want to test for lower case chars before doing the UPDATE, using any case-sensitive collation:

UPDATE CONTACT
SET COUNTRY = UPPER(COUNTRY)
WHERE COUNTRY...

You can also get that if you don't include the schema name when referencing tables.  For example:

SELECT ...
FROM table1

SQL will often generate a separate plan...