20s. Donâ€™t be tempted by the credit card offers that will surely come your way (you wonâ€™t get offers if youâ€™re under the age of 21â€”card issuers are not allowed to offer credit unless you can provide proof of a means to pay the bill or an adult over 21 cosigns your application). If you decide to get creditâ€”and you have a means to pay the billâ€”donâ€™t get more than two cards. Any more than that and youâ€™ll have trouble keeping up with payments. Youâ€™ll also be tempted to charge more.