TSQL - Age in years, Sql Server

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with data(StartDateTime, EndDateTime) as (
    select cast(d1 as datetime), cast(d2 as datetime) from (values
        ('2006-10-10 16:08:13.523', '2008-04-11 00:00:00.000'),
        ('2016-05-11 13:03:48.093', '2016-06-16 00:00:00.000'),
        ('2016-08-01 12:44:42.990', '2016-08-01 00:00:00.000'),
        ('2016-05-20 17:33:27.957', '2016-05-25 00:00:00.000'),
        ('2006-10-19 21:41:41.350', null),
        ('2006-01-09 11:42:43.000', null),
        ('20120229', '20130228'), ('20160101', '20161231'), ('20150101', '20160101')
    ) t(d1, d2)
)
select
    StartDateTime, EndDateTime,
    case
        when datepart(year, coalesce(EndDateTime, getdate())) > datepart(year, StartDateTime)
        then
            datediff(year, StartDateTime, coalesce(EndDateTime, getdate())) -
            case
                when
                    EndDateTime is not null and
                        datepart(month, EndDateTime) < datepart(month, StartDateTime)
                     or datepart(month, EndDateTime) = datepart(month, StartDateTime)
                    and datepart(day,   EndDateTime) < datepart(day,   StartDateTime)
                    and not -- catch the leap day condition
                    (
                            datepart(month, StartDateTime + 1) > datepart(month, StartDateTime)
                        and datepart(month, EndDateTime + 1)   > datepart(month, EndDateTime)                    
                    )
                then 1
                when
                    EndDateTime is null and
                        datepart(month, getdate()) < datepart(month, StartDateTime)
                     or datepart(month, getdate()) = datepart(month, StartDateTime)
                    and datepart(day,   getdate()) < datepart(day,   StartDateTime)
                    and not -- catch the leap day condition
                    (
                            datepart(month, StartDateTime + 1) > datepart(month, StartDateTime)
                        and datepart(month, getdate() + 1)     > datepart(month, getdate())
                    )
                then 1
                else 0
            end
        else 0
    end as Age
from data;

View schema
Execution time: 0 sec, rows selected: 9, rows affected: 0, absolute service time: 0,16 sec