This would work too
As an explanation, I use datediff to get the number of days, divide that by the correct number of days in a year (keeping track of leap years, there are only 97 in every 400 years) and then FLOOR the result, thus making sure it doesn't get rounded up accidentally.
CREATE FUNCTION [dbo].[GetAge]
RETURN FLOOR(DATEDIFF(dd, @Geburtsdatum, CURRENT_TIMESTAMP)/365.2425)