MS SQL Server function to get a string in natural language with a person's age (newborns to elderly).

GetStringAge.sql

CREATE FUNCTION [dbo].[Getagestring] (@birth_date datetime)
RETURNS varchar(80)
AS
BEGIN
  DECLARE @age int

  SELECT
    @age = DATEDIFF(yy, @birth_date, GETDATE()) - CASE
      WHEN
        DATEADD(yy, DATEDIFF(yy,
        @birth_date, GETDATE()
        ), @birth_date) > GETDATE(
        ) THEN 1
      ELSE 0
    END

  DECLARE @meses int

  SELECT
    @meses = DATEDIFF(mm, @birth_date, GETDATE()) - CASE
      WHEN
        DATEADD(mm, DATEDIFF(mm,
        @birth_date,
        GETDATE()
        ), @birth_date) >
        GETDATE(
        ) THEN 1
      ELSE 0
    END

  
  IF @age >= 18
    RETURN CAST(@age AS varchar) + ' Years' 
  
  IF @age >= 2
  BEGIN
    DECLARE @mesesParciales int

    SELECT
      @mesesParciales = (DATEDIFF(mm, @birth_date, GETDATE())
      - CASE
        WHEN
          DATEADD(mm, DATEDIFF(mm,
          @birth_date, GETDATE
          ()
          ), @birth_date) >
          GETDATE(
          ) THEN 1
        ELSE 0
      END
      ) %
      12

    RETURN CAST(@age AS varchar) + ' Years '
    + CAST(@mesesParciales AS varchar)
    + ' Months'
  END

  IF @meses >= 4
  BEGIN
    RETURN CAST(@meses AS varchar) + ' Months'
  END

  DECLARE @dias int

  SELECT
    @dias = DATEDIFF(dd, @birth_date, GETDATE()) - CASE
      WHEN
        DATEADD(dd, DATEDIFF(dd,
        @birth_date, GETDATE()
        ), @birth_date) > GETDATE(
        ) THEN 1
      ELSE 0
    END

  IF @dias >= 28
  BEGIN
    DECLARE @semanas int

    SELECT
      @semanas = DATEDIFF(ww, @birth_date, GETDATE()) - CASE
        WHEN
          DATEADD(ww,
          DATEDIFF(ww,
          @birth_date, GETDATE()
          ), @birth_date) >
          GETDATE(
          ) THEN 1
        ELSE 0
      END

    RETURN CAST(@semanas AS varchar) + ' Weeks'
  END

  IF @dias >= 5
  BEGIN
    RETURN CAST(@dias AS varchar) + ' Days'
  END

  DECLARE @horas int

  SELECT
    @horas = DATEDIFF(hh, @birth_date, GETDATE()) - CASE
      WHEN
        DATEADD(hh, DATEDIFF(hh,
        @birth_date,
        GETDATE()
        ), @birth_date) >
        GETDATE(
        ) THEN 1
      ELSE 0
    END
  RETURN CAST(@horas AS varchar) + ' Hours'
END