Young runners should run as much as they want to. There’s no evidence to support the contention that running as a growing teen will stunt growth. If a teen enjoys her running, is doing it because she wants to, and is continuing to improve while remaining injury-free, she’s on the right track. I ran a marathon while in high school and didn’t burn out on the sport, as is so often feared for young runners. Thirty years after that marathon, I’m more in love with running than ever.