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.
The other age group whose average credit score skews lower are those below the age of 30. One contributing factor to this could be the limited access to credit this age group faces. Following the 2009 CARD Act, it became significantly harder for 18 to 21 year olds to open new credit card accounts.