Why had Elihu waited before speaking to Job?

Answer: The others were older than he

Elihu waited because Job's friends were older; he was young and timid, but spoke when they had no more answers.

“Now Elihu had waited to speak to Job because the others were older than he. But when he saw that the three men had no further reply, his anger was kindled. So Elihu son of Barachel the Buzite declared: “I am young in years, while you are old; that is why I was timid and afraid to tell you what I know.”— Job 32:4-6 (BSB)