How 'bout this?

declare @dob as datetime

select @dob = '6/21/1968'

declare @now as datetime

declare @year_diff as tinyint

select @now = getdate()

select @year_diff = datediff(year, @dob, @now)

if DateAdd(year, @year_diff, @dob) > @now

select @year_diff -1

else

select...