Question: What age do Orthodox Jewish men start wearing black hats? Is there a hard rule or is it down to custom?
Answer: The hat is completely a custom and different groups wear different types of hats to reflect where they are from or just to associate with a specific group. The age is usually Bar Mitzvah to start wearing it. The Jewish legal reasoning comes from the Code of Jewish Law which teaches that we should have head coverings at all times and even mentions a double head covering while praying.
I hope this helps.
Be Well,
Rabbi Litt