You could try the following, substituting in relevant dates in @d1 and @d2:
declare @d1 datetime = '3-jan-2000'
    ,    @d2 datetime = '2-may-2000'

select isnull(cast(nullif(case    when datepart(month, @d1) > datepart(month, @d2)
                                then datediff(year, @d1, @d2) - 1
                                else datediff(year, @d1, @d2)
                         end, 0) as varchar)
             + case    when (select case when datepart(month, @d1) > datepart(month, @d2)
                                then datediff(year, @d1, @d2) - 1
                                else datediff(year, @d1, @d2)
                         end) > 1 then ' Years, ' else ' Year, ' end, '') +
     isnull(cast(nullif(case when datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2) < 0
                                then (datediff(month, @d1, @d2) - 1) % 12
                                else datediff(month, @d1, @d2) % 12
                         end, 0) as varchar)
            + case when (select case when datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2) < 0
                                then (datediff(month, @d1, @d2) - 1) % 12
                                else datediff(month, @d1, @d2) % 12
                         end) > 1 then ' Months, ' else ' Month, ' end, '') +
     isnull(cast(nullif(case when datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2) < 0
                                then datediff(day, dateadd(month, datediff(month, @d1, @d2) -1, @d1), @d2)
                                else datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2)
                         end, 0) as varchar)
            + case when (select case when datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2) < 0
                                then datediff(day, dateadd(month, datediff(month, @d1, @d2) -1, @d1), @d2)
                                else datediff(day, dateadd(month, datediff(month, @d1, @d2), @d1), @d2)
                         end) > 1 then ' Days' else ' Day' end, '')