SQL Server 2017

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create table users (
 userid int, 
 registrationdate date
);

insert into users values 
(42, DATEADD(day, -3, DATEADD(month, -1, DATEADD(year, -4, GETDATE()))));
1 rows affected

CREATE FUNCTION dbo.fnGetDateInterval 
(
   @FromDate DATE, @ToDate DATE
)
RETURNS NVARCHAR(40)
WITH SCHEMABINDING
AS
BEGIN
  DECLARE @Years INT, @Months INT, @Days INT;
  DECLARE @tmpFrom DATE;
  SET @Years = DATEDIFF(YEAR, @FromDate, @ToDate)
      - IIF(@ToDate < DATEADD(YEAR, DATEDIFF(YEAR, @FromDate, @ToDate), @FromDate), 1, 0);
  SET @tmpFrom = DATEADD(YEAR, @Years , @FromDate)
  SET @Months =  DATEDIFF(MONTH, @tmpFrom, @ToDate)
      - IIF(@ToDate < DATEADD(MONTH,DATEDIFF(MONTH, @tmpFrom, @ToDate), @tmpFrom), 1, 0);
  SET @tmpFrom = DATEADD(MONTH, @Months , @tmpFrom)
  SET @Days =  DATEDIFF(DAY, @tmpFrom, @ToDate)
      - IIF(@ToDate < DATEADD(DAY, DATEDIFF(DAY, @tmpFrom, @ToDate), @tmpFrom), 1, 0);
  RETURN CONCAT('{', '"years":', @Years, 
                     ',"months":', @Months, 
                     ',"days":', @Days, '}')
END;
DECLARE @UserId INT = 42;

SELECT CONCAT(days, ' days ', months, ' months ', years, ' years') AS DaysMonthsYears
FROM users usr
CROSS APPLY OPENJSON(dbo.fnGetDateInterval(usr.registrationdate, GETDATE())) 
            WITH (years  INT '$.years',
                  months INT '$.months', 
                  days   INT '$.days') AS js
WHERE  usr.userid = @UserId;


DaysMonthsYears


3 days 1 months 4 years