I would contact the Airline company and ask them, I wouldn't take someones word for it and find out later they were wrong.
For international travel, a passport is required, even for children under 18. For travel within the US with an adult, no travel document is required except the boarding pass. However a picture ID may be helpful. Traveling unaccompanied may require special arrangements; almost always for children under 12, but it depends on the airline whether 15 is old enough, and whether there are connections.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them