Children and the Half Marathon (and Marathon)
Let's first take a look at what may or may not be appropriate for a developing body. First, let's consider the game of soccer (football, except in the U.S.A.), that game involves 90 minutes of play and the athletes are constantly moving, switching between a jog and a sprint. Depending on the team and coach, substitutions may or may not be made. Most parents would not question this, after all it's a game; but they will question longer runs. Are they different? Absolutely, but is running meaningfully more demanding? No, and probably less so. Without the addition of those bursts of speed required in soccer, the loads on the body are generally less.
If we assume a comfortable pace for a child is between an 8 and 10 minute mile (roughly 5 to 6 minutes per kilometer), then we can figure finish times for a half marathon in the 1:25 to 2:10 range. To me, these are reasonable speeds for kids, but with some caveats. It's here that we get into some of the real concerns about the propriety of this kind of activity.
Let's first look at prepubescent children. In many ways, I worry less about these kids than I do with those who are into puberty. First, keep in mind that young kids do not sweat well, so in conditions of high heat, serious consideration should be made to pull them from the event (I will use the terms event and race interchangeably here, but will often select one or the other to convey a message. In this case, I generally don't consider it appropriate for kids to race this kind of distance at these ages, so I choose the term event.) If they are not pulled from the race, then make sure there is a plan for external cooling. This means you should plan a strategy for evaporative or other cooling of the athlete through use of water and or ice to keep their core temperature from rising. I won't describe the process in this post, but may cover it later.
Second, prepubescent children tend to be very light, normally about 90 pounds (40 kg) or less. This means that loads on joints and connective tissues are fairly low and the risks of injury are concomitantly low. Third, children are often pretty attuned to their bodies and will stop running in case of injury or pain, unless adults have skewed things with messages like "finish what you start" or "Billy finished one, why can't you?" Kids should be encouraged to listen to their bodies. Whether an "I'm tired" message should be heeded is more debatable, but a "my knee hurts" message should unquestionably be listened to.
Third, the child should be enjoying the activity. It's here that things get dicier especially in regard to the question of what is enjoyment. I think most of us have struggled through something and afterwards felt good about the achievement. To me, with kids, this is NOT an acceptable level of enjoyment. The activity itself should be enjoyable. Kids should truly be having fun out there, and if they get a feeling of achievement, it should be coming from inside them. I know that many children get a lot of joy at beating grown-ups at something, and this, to me, is about as far as the achievement thing should go.
When we get to children experiencing puberty, I think we need to be a bit more careful in a number of ways. First, growth occurs at different rates in different parts of the body. Second, as weight is added, related tissues don't always develop initially with the required strength level to handle the bigger loads they can be subjected to. Thus, in my opinion, some easing of intensity MAY be appropriate. I think this is best handled by conversation with the athlete, but because kids often desire to be aloof from parents at this time, a more "arm's length" relationship, whether a coach, medical professional, teacher, aunt or uncle, or adult mentor may be able to get around the wall that's often there.
Girls, especially, face issues during this time. The female athlete triad, which is comprised of disordered eating, menstrual disturbances (irregular periods and/or amenorrhea), and bone loss issues (osteopenia and osteoporosis). Don't assume, however, that if your athlete is a boy, you get a pass here. Male athletes are subject to at least two of these conditions. My first piece of advice is to NEVER tell ANY athlete that losing weight will help their performance. Instead, teach them to eat healthy foods to satiety. this means large amounts of green vegetables, especially leafy ones; large amounts of complex carbohydrates (studies show that most top athletes get roughly 80% of their calories here, mostly from grains); and adequate meat to fill their protein needs. (Vegetarians and vegans can fulfill this need through appropriate non-meat sources, but it adds some complexity and potential for concern.) Athletes in their teens also are often dealing with body image issues, and not just girls, so following the old aphorism, "If you don't have something nice to say, don't say anything at all" is a wise course.
The Boston Marathon has a minimum entry age of 18. Many other marathons have a minimum of 16 or 17, and I think this is wise. There are stories of kids at Boston (and occasionally at other races) getting caught up in the excitement of the race and running in with it, occasionally for the bulk of the race. It's not the same as running with an entry, but it also removes much of the baggage that can cause wrong choices to be made. While kids can get razzed by their friends if they bail out, most are not too harsh about it. It can be a good entree into the sport and without the "have to finish" stigma. While I would not suggest this for adults, it's a way for kids to get a sense for what long distance running truly is about (and if there is a minimum age for the race, it's a way of skirting that restriction). If you are considering this, you should make a plan about what they'll do to get home, and they should be carrying a few bucks to help execute the plan and or ease their way. Also, keep in mind that the kids are at the mercy of adults out there, and while it's appealing to think "all runners are good people", unfortunately it's probably not so. Make sure they know to stay in public places and, if possible, with groups of adults or authority figures.
My general feeling about the marathon is that athletes should not run it until they meet 2 criteria, 1) they have stopped growing rapidly in height for at least a year and 2) they are sufficiently trained. (I'd say a good guideline is 35+ miles a week for at least several months including at least one weekly long run of at least 15 miles for a couple of months prior to the race.)
So, to sum up, I think the half-marathon, on an occasional basis is OK for kids. The marathon should not be for kids, though maybe for young adults. I'd say that if you're responsible for a child, the best bet is to keep their racing limited to 5k's and 10k's with perhaps an occasional 15k or half-marathon tossed in, especially if they are well trained for it.