W A Mozart was composing at the age of 4 years. I personally know of no younger composers.
Another answer:
This is not the sort of question that can have an exact answer. In the case of most composers there is no hard-and-fast indication of how old they were when they first wrote something. You also have to bear in mind that there may not be a clear dividing line between childish jottings and 'composing' - how do you know when the one becomes the other?
The youngest symphony composer is Wolfgang Amadeus Mozart. He began composing at the age of six and wrote his first symphony at the age of eight.
Mahler (his 7th symphony).
Benjamin Britten
Franz Joseph Hayden.
Beetoven
William Grant Still
Mahler (his 7th symphony).
John Corigliano is the composer for the Baltimore Symphony Orchestra. Marin Alsop and Mark O'Connor are also composers for the Baltimore Symphony Orchestra.
Benjamin Britten
Beethoven was the composer and arranger of the original version of his Fifth Symphony.
Franz Joseph Hayden.
Beetoven
Pastoral symphony is composed based on village environment. It is Beethoven's 6th symphony. It is the only symphony by the composer with five movements.
William Grant Still
Beethoven.
Many composers wrote a 6th symphony. This question is meaningless without knowing which composer.
symphony in c and no. 4
That would be Ludwig van Beethoven.