Write SQL query to solve given problem: At what age did Michael Fred Phelps, II join the Olympics?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.age FROM person AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.person_id WHERE T1.full_name = 'Michael Fred Phelps, II' ORDER BY T2.age LIMIT 1