According to Creditnet, the minimum legal age to obtain a credit card in the United States is 18. However, there are other credit options for those under 18, including having a parent co-sign a line of credit or purchasing prepaid cards.
About.com explains that being 18 does not necessarily guarantee credit card approval. A person must first be able to prove that he has sufficient income to repay any debt he incurs through the credit card within the lender's time frame.
Creditnet notes that many parents open a line of credit and add their minor child as an authorized user of the account until the child is old enough to get credit of his own.