At the age of 38, Tchaikovsky composed his Violin Concerto. 
At 33, Rachmaninov signed his Symphony No. 2. 
Andrea Obiso became Concertmaster of the Orchestra of the Accademia Nazionale di Santa Cecilia at 30 years old. 
At 30 years old, Emmanuel Tjeknavorian is in his second season as Music Director of the Orchestra Sinfonica di Milano. 

If what was published a few days ago in an article in the Corriere Salute is true, namely that

then the fifth concert of the Orchestra Sinfonica di Milano shows us how, before that age, four thirty-year-olds (two on stage, two authors of the compositions) can reach unusual peaks.