Generally, age cannot be used to make credit decisions; however, it may be considered in certain circumstances. For example, a creditor may use an applicant’s age as part of a valid credit scoring system (so long as it does not disfavor applicants 62 years old and older).
In addition, if you are under 21 years old, the card issuer cannot issue a credit card to you unless you can show an independent ability to make the minimum periodic payment on the account or someone at least 21 years old guarantees or otherwise agrees to be liable on the account. The guarantee may come from anyone at least 21 years old with the financial ability to make the payments and does not have to come from your parents.
If you have friends under 21 who ask you to help them get a loan, consider that if they do not pay, you would be obligated to do so, and if you do not, it will likely hurt your credit history. If they do not make timely payments, this also may appear on your credit report.