They will quote an hourly rate for their time and tell the customer that they work on a time and materials basis. That means X many dollars per hour of time, plus the cost of parts and supplies. Most shops also charge for research time, shipping of parts, recycling, rags and so forth.
You must be 18 years old to get a credit card by yourself. While you can legally get a credit card as young as age 18, the law requires you have your own steady source of income to qualify for a credit card. Unfortunately, a monthly allowance from your parents doesn't count. It's for your own good.
Everyone is correct, it is age 10. However there are certain situation that may warrant the child to get a card before the age of 10. If the child lives with their custodial parent part of the time, then they are still eligible for medical/dental benefits and can have their ID before the age of 10.