Age calculation with specified date

SmithToronto 

I think I resolved the problem, code is here

CREATE FUNCTION CalcAge (@From AS Date, @To AS Date)
RETURNS VARCHAR(20)
BEGIN
	DECLARE @y int
	DECLARE @m int
	DECLARE @d int

	SET @y = DATEPART(year, @To) - DATEPART(year, @From)
	SET @m = DATEPART(month, @To) - DATEPART(month, @From)
	SET @d = DATEPART(day, @To) - DATEPART(day, @From)
	SET @d += 1

	IF @d < 0
	BEGIN
		SET @m -= 1
		IF DAY(@From) > DAY(EOMONTH(@To, -1))
			SET @d += DAY(EOMONTH(@From))
		ELSE
			SET @d += DAY(EOMONTH(@To, -1))
	END

	IF @m< 0
	BEGIN
		SET @y -= 1
		SET @m += 12
	END

	RETURN FORMAT(@y, '#00') + ':' + FORMAT(@m, '00') + ':' + FORMAT(@d, '00')
END
GO

I made it as a function, you can test it with script like below

DECLARE @i INT
DECLARE @b DATE
DECLARE @e DATE
SET @i= 0
SET @b= '2023-03-31'
SET @e= '2023-03-31'
WHILE (@i<368)
BEGIN
	PRINT CAST( @b AS varchar(20)) + ' to ' + CAST( @e AS varchar(20)) + ': '+ dbo.CalcAge(@b, @e)
	IF(DAY(@e) = DAY(EOMONTH(@e))) PRINT '-----------------------------------------'
	SET @e = DATEADD(day, 1, @e)
	SET @i += 1
END

For your sample, this function will return result '53:11:30', different to your wish. That's think this way, we usually treat same day as a whole month(to your method, it is the day before birthday). For example, when birthday is 1970-09-18, we say 2024-09-17 is 54 years old, so 2024-09-16 is 1 day left to 54 years old, the last whole month to 16th/9 is Aug., and Aug. has 31 days, so 54 year decrease 1 day should by 53:11:30.