There is no characteristic in the FICO scoring model that penalizes you or gives you points because of your age. What is age-related and does affect your FICO score is the average age of the accounts on your credit report and how old your credit history is.
Therefore, while an eighteen-year old is not penalized for being young, in the sense that most eighteen-year-olds may have only one recently opened credit card, their credit histories will be very recent and therefore their credit scores will be lower. Parents may offset this reality by placing their children as authorized users on their accounts when their children are younger teenagers.
Lack of credit history is also a main reason why people who avoid the credit world or are new to the U.S. have trouble opening a credit card: they have no established credit history. Secured cards are a good option for people who are just starting out in the credit world, including young people.