Final answer:
A musical prodigy can compose a symphony and other works often before the age of 10, as demonstrated by the likes of Mozart who wrote his first symphony at eight.
Step-by-step explanation:
The minimum age at which a prodigious composer had already composed a symphony and numerous other works is typically less than 10 years. Many musical prodigies have demonstrated extraordinary talent at very young ages, with some composing substantial works before reaching their teens. For example, Wolfgang Amadeus Mozart composed his first symphony at the age of eight. Moreover, the education during the time period when prodigies like Mozart lived often included music from a very young age, contributing to their early development in the field.