What's the meaning of 'at the age of five'?
Here is the whole sentence:

He started composing at the age of five and wrote more than 600 pieces of music.

Question

What's the meaning of 'at the age of five'?
Here is the whole sentence:

He started composing at the age of five and wrote more than 600 pieces of music.

Answer

At the age of five = when he was five years old.

He started composing music when he was five years old.

Answer

He started composing when he was 5 years old