British Council's page provides a useful parallel:
“I work in Italy” Reported speech: He told me that he works in Italy.
The tense isn't changed because the fact is still true. The tense shifting is a technique or a recipe to easily convert sentence from direct to indirect speech, but the final sentence should still follow general rules for tense.
Let's construct a timeline here:
- He was 13 years old
- He was discovered by a producer
- He was still 13 years old
- He spoke to a journalist about 2
1 and 3 are general circumstances in the past. They are not in relation to events 2 or 4. However, 2 happened before 4, so we use past perfect for 2.
He said that a producer had discovered him.
Even if we put aside the rules of speech transformation, this sentence follows general rules of tenses.
Now, when we add in the age, what we are adding is a general truth in the past, so it takes simple past tense. His age was 13 both before, during and after the producer discovered him.
He said that a producer had discovered him when he was 13.