SQL state for calculating age

New to SQL server have two columns BIRTHDAY and AGE. Is there a statement to calculate the age from the BIRTHDAY column and enter it into the AGE column, any help would be appreciated.

An AGE column is going to be out of date (at some point) isn't it?

If you need AGE probably better to have either a calculated column (rather than a column with a persistent value), or a VIEW that also includes the calculation.

	[AGE] = DATEDIFF(Year, BIRTHDAY, GetDate())
					, DATEDIFF(Year, BIRTHDAY, GetDate())
					, BIRTHDAY)
				> GetDate() THEN 1
			ELSE 0

Thank you that got me started