Title III of the Credit Card Accountability Responsibility and Disclosure ("Credit CARD") Act of 2009[1] and the CFPB's Reg. Z[2] which implements it. It amended the Truth In Lending Act to prohibit a lot of the "sign up for a credit card and get this free poster" marketing that had been happening on college campuses. It also requires that card issuers evaluate a consumer’s ability to pay before opening a new credit card account or increasing a credit limit. That requirement effectively stopped the issuance of credit cards to full-time college students without a co-signer. It also had the unintended side effect of making it much more difficult for stay-at-home parents to get a credit card without their spouse as a co-signer because the regulation only permitted card issuers to consider the applicant's income/assets. Reg. Z was amended a few years ago so now card issuers only need to evaluate someone's ability to pay if they're under the age of 21.

[1]https://www.ftc.gov/sites/default/files/documents/statutes/c...

[2]https://www.consumerfinance.gov/policy-compliance/rulemaking...