My grandfather enlisted in the Navy at 16 years old and served on the USS West Virginia during WWII. Was underage enlistment like this common in the U.S.?
Edit:
If it helps answer the question, he was raised in Nashville, TN and by all accounts grew up rather poor.
How would someone go about getting enlisted underage? Would it require a fake ID or just were there enlistment people who didn't care?
Most evidence of this occurring is likely to be anecdotal, due to the illegal nature of the practice. From the evidence, it appears that getting away with it only required someone willing to provide an affidavit that the person is of age.
Audie Murphy got in by having his sister claim he was old enough.
If memory serves me correctly (been a while since I read his book) Colonel Hackworth found a homeless person to claim he was his father. He later submitted a form to have his birthdate corrected, and no disciplinary action was taken.
More anecdotal evidence of how it was done.