Answer:  6 months, according to FICO.

From GoBankingRates:

Teens will have a credit score as soon they keep one credit account open for about six months, according to FICO. Though they might not realize it, the credit history teenagers start to develop will have a huge impact on their lives, from the interest rate they receive on their first auto loan to the kind of credit card they can obtain in college. Because of this, parents need to make sure their children are well-informed about the benefits and pitfalls of managing credit, and how to build a great credit score.

About the Author

Tim Ranzetta

Tim's saving habits started at seven when a neighbor with a broken hip gave him a dog walking job. Her recovery, which took almost a year, resulted in Tim getting to know the bank tellers quite well (and accumulating a savings account balance of over $300!). His recent entrepreneurial adventures have included driving a shredding truck, analyzing executive compensation packages for Fortune 500 companies and helping families make better college financing decisions. After volunteering in 2010 to create and teach a personal finance program at Eastside College Prep in East Palo Alto, Tim saw firsthand the impact of an engaging and activity-based curriculum, which inspired him to start a new non-profit, Next Gen Personal Finance.