According to an article in LiveStrong:
According to Running Times, the maximum recommended distance
for young runners varies by age. Kids younger than age 5 can
participate in designated kiddie events but should focus primarily on
running as part of natural play. By age 5, kids are ready for light
training leading up to a mile-long run. Preteens and early teens can
manage 5K runs, and some early teens are ready for a 10K. Older teens
are generally ready to compete on an adult level.
Rapidly growing bones and muscles are increased risks for injuries
such as shin splints. Undiagnosed medical conditions such as asthma or
heart defects can affect a child's ability to participate in
high-impact sports. Schedule your child for a full sports physical
before beginning serious training. Follow the doctor's recommendations
for training schedules, rest breaks and cross training in other
sports. Have any injuries or persistent pain thoroughly checked. Make
sure your child drinks plenty of water or sports drinks, warms up
thoroughly before each workout, and cools down appropriately at the
end of each session.
Runner's World also discussed running and age:
Framing marathon performance as an age issue is wrong, says Terrence
Mahon, coach to Olympic marathoners Deena Kastor, Ryan Hall, and
Mahon's wife, Jen Rhines. What's key, says Mahon, isn't chronological
age but athletic age—a runner's history of aerobic training—and that
goes for elites and nonelites alike.
In the past, coaches worried that turning to the marathon too soon
would unduly shorten a runner's career. [Coach Brad] Hudson ran his first marathon
at age 12. "I think it hurt my career to go to the marathon so young,"
says Hudson, who set his PR (2:13) at 23. Today, he doesn't advise
kids to begin racing marathons as young as he did, but he says that
high school and collegiate runners can handle high volumes of training
if they increase their mileage gradually and take steps to avoid
What successful young marathoners like Wanjiru have in common with
older ones like Tomescu–Dita is a solid mileage base. "There are
athletes out there running marathons at 21, but if you look at most of
them, you'll see they've had a high training volume for a good many
years before they've run the event," Mahon says. "Ryan Hall and Dathan
Ritzenhein were running 90 miles per week in high school, so for them
to make the transition to 100—mile weeks wasn't so hard."
Obviously, young runners outside the elite ranks don't need to attain
that kind of mileage. But the same principle applies—to prepare for
the marathon, first build a strong endurance base, then adopt a
graduated training program that builds to the marathon distance, says
Jack Daniels, an exercise physiologist and head distance coach at the
Center for High Altitude Training at Northern Arizona University in
In the end, age is probably not the biggest factor - it's the amount of distance training that the athlete's body is conditioned for.