The third solution returns "51" for DOB 08/21/1957.

select DATEDIFF(yy, '08/21/1957', GETDATE()) - CASE WHEN DATEPART(mm, '08/21/1957') >= DATEPART(mm, GETDATE()) AND DATEPART(d, '08/21/1957') >= DATEPART(d, GETDATE()) THEN 0 ELSE 1 END

Clearly, this is INCORRECT.