Some people will tell you that kids are never too young to start getting fit. While this is generally true, you need to place limits on 13-year-olds when it comes to running on a treadmill. Left to their own judgement, young teens may over-train and injure themselves. Understand the recommended running guidelines for young athletes and discuss them with your child.
Session Limit
You should limit your 13-year-old's treadmill sessions, but the key thing to limit is distance, not time. The International Association of Athletics Federations recommends that 13-year-olds run no more than 10K at a time -- approximately 6.2 miles. At a fast pace of six minutes per mile, this works out to just over 37 minutes, but at a slower pace of 11 minutes per mile, it works out to about an hour and eight minutes.
Total Weekly Time
The IAAF also recommends that 13-year-olds should not run more than 20K, or 12.4 miles, in a week. This means that if your child runs 10K in one session, her remaining sessions should not add up to more than 10K. The time she will spend on the treadmill again varies with her speed, but even at a slow pace of 11 minutes per mile, she won't be running more than two hours and 17 minutes per week.
Number of Sessions
The IAAF also caps the total number of running sessions that a 13-year-old should engage in. Your child shouldn't run more than three times a week, even if she hasn't hit her weekly mileage limit. So if your child ran a mile on Sunday, 2 miles on Tuesday and 3 miles on Thursday, she shouldn't run again until the following week, even though she's 6.4 miles shy of her weekly mileage limit.
Considerations
According to the Road Runners Club of America, 13-year-olds should be ready to start systematic training. But your child may not be ready for competitive training, because this is gender-dependent. A 13-year-old girl is ready to start competitive training -- in fact, girls should be ready by age 12. But boys won't be ready to begin competitive training until age 14. Consult your child's pediatrician to see whether your child is ready. A doctor can also evaluate your child for potential health problems that could further limit treadmill training.