How to work out age in SQL Server (and why DATEDIFF(year) lies)

DATEDIFF(year, dob, today) counts New Year’s Eves, not birthdays. Someone born 31 December 2000 is “26” on 1 January 2026. They are 25.

The honest age

SELECT customer_id, first_name, date_of_birth,
       DATEDIFF(year, date_of_birth, GETDATE())
         - CASE WHEN DATEADD(year, DATEDIFF(year, date_of_birth, GETDATE()), date_of_birth) > CAST(GETDATE() AS date)
                THEN 1 ELSE 0 END AS age
FROM Customers;

Work out the year gap, then subtract one if this year’s birthday is still in the future.

Age in days or months

SELECT DATEDIFF(day, date_of_birth, CAST(GETDATE() AS date)) AS age_days;

Days are honest. Months have the same boundary issue as years.