Write SQL query to solve given problem: What is the average age of the sales agents in the company by 12/31/2009?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'