Not everyone is required to file an income tax return each year. Generally, if your total income for the year doesn't exceed certain thresholds, then you don't need to file a federal tax return.
TurboTax is asking if you son was not required to file but he had some taxes withheld so he is filing to get a refund. Please check the link for Income thresholds and for information on who needs to file.
Does Everyone Need to File an Income Tax Return?
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