What is the average age of the sales agents in the company by 12/31/2009?
average age as of 12/31/2009 = AVG(SUBTRACT(2009, year(BirthDate));
SELECT AVG(2009 - STRFTIME('%Y', T2.BirthDate)) FROM Person AS T1 INNER JOIN Employee AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID WHERE T1.PersonType = 'SP'