Yes, it matters a lot.
Answers depend upon who you are talking to and what their experience is. Lots of people can talk about how to coach an athlete but few can manage young athletes and fewer still have any sense at all about how to manage the family. In fact most of the advanced coaches I know would rather never talk to the family except to get a check. It is a highly advanced skill. Further, few youth mtb coaches know how to manage anything other than a pure drive to victory in the short term. It IS a new field after all.
The candle which burns twice as bright.....
I will assume we are talking about a boy. We need to look at why this athlete needs to be bridled in. At the age of 13-15 growth becomes a huge factor as bones, muscles, soft tissues, and the nervous system all change at different rate. Growing pains are a soft sign of the conflicts caused by these events. At the same time the kid is accelerating his use of his entire body with the facile commitment of a child at play into the demands of the first adult-like work efforts. As he has not yet found his limits and believes he has none as his psyche is rapidly changing, the whole a mix of new hormone levels. Stressing systems at this age can cause huge amounts of damage through over-use or simply exceeding his own skill level and that of the racers around him. I have seen damage from both sorts of use where athletes hurt themselves by over-use while growing or crash by exerting themselves into situations they cannot manage yet.
So your job is to appreciate the degree to which this kid needs to be reigned in. Once he is into years 16 and up the growth issue is reduced but not eliminated. You can make general predictions about his needs by comparing his size to the size of the adults in his family. You can also fine-tune this understanding by getting a sense of what kinds of racehorses this family produces. I have seen families who simply, as a way of life, have a very high tolerance for physical and mental stress and do quite well with it. Additionally some families breed rawboned specimens who seem to elude the growth injury problems though turning these kids loose can bring crash injuries as described above. In a family accustomed to producing athletes how we handle the phenom can be quite different. For all the fear we have as responsible developers of talent and concern for over-use and burn-out, for some family lifestyles it is just not a problem. For some athletes it is deadly.
Additionally, you have to look into where the motivation for participation in this sport, and in this level, is coming from. The choice to perform in this sport must, at all times, come from the athlete and not the parents. Once that is understood then we have to deal with issues of success, the value placed upon it and how it is defined. Early success can creat a false sense of urgency or can feed a monster of excitement that thrives within an entire family , dreams runnning rampant, and this has to be appreciated as well. Early successes, read: victory, can pervert an appreciation for any other kind of overall success. Sure everyone loves to win but the richness of the experience, good health, cameraderie, academic efficacy, in the context of a healthy teen social life can suffer at the cost of winning. It is a heady price to pay for a child. And these ARE children, make no mistake, not miniature Olympians.
The mitigation of this athete is a two step process but both of these steps depend heavily on the regard the family and the athlete have for the coach:
1) Help the athlete and the family to understand the value of reducing the overall intensity.
2) Impliment strategies to structure the variety of efforts.
If you have determined the source of the motivation in this system then you can focus your effort. If, as we all hope, the main motivation comes from the child then we can deal with issues concerning a typical adolescent drive to express newfound power without the incumbent skills to manage it. It is a classic expression of a teens need for firm and fair limits in service of lifelong goals for a good life. These parallel parenting responsibilities in the best of worlds. The better your work is with your family the easier it is going to be to guide your young racer.
As a coach you are the authority figure where this sport is concerned. With consideration of your athletes needs you will describe the program in the context of workouts and rest periods over time and the execution of what you feel to be a reasonable number of races per season, a space of time which you also will define. Post season activities will specifically prescribe substantial amounts of time off the bike. I know that this sounds a bit severe but that is your job. It is not a method which works with adults but remarkably well with teens who are still operating in a teacher-pupil mode no matter how much they are struggling to independence. Besides, rest, outside recreation, and other enrichments are the best solutions to avoid burn-out.
The great challenge is that your success, that is, the health of the athlete in a consistantly satisfying competitive expression, can always be challenged to have been greater or more productive after the fact. What we hope people will never see is that those efforts allowed for greater or more productive results end in damage and real failure. In my years of working with teens and in healthcare respected collegues have helped me learn that if you make mistakes with people in your care, especially children, make them on the conservative side. At bottom: where is the urgency. These are kids. This is a much longer game that teh families realize with consequences you, not they, can forsee.
I am not one to promote excursions into other aspects of cycling except to dabble. It only leads to more time on a bike (see substantial amounts of time off the bike above.) I have also seem some pretty substantial damage when a highly skilled athlete jumps into another aspect of cycling with the will, not the skills, absent the ability to see the difference. I do encourage participation in other kinds of sports like swimming and running, especially because they serve my purpose in conditioning the athlete without the risk of soccer, football or rugby. This is my bias.
It has been my great fortune to enjoy the confidence of the families of the athletes with which I have worked. I have also developed teen athletes on a par with some of the best you will see though they weren't great when I got them. I transitioned them from other sports, kept them healthy, and resisted pushes from them (which had some root from a Dad or two) to fine results. When you have kids like this they are already winning so what are the next goals. More winning? Winning all the time? Winning everything?
It takes a lot to win; skill, power, luck, talent, and brains. In this case you are the brains for the racer and the family and your goal is to manage a talent, perhaps greater than your own, and hand him off in good form, to the next brain at the next level. Meanwhile your phenom has a life in the busom of his family and you are a guide to them all. You are dealing with a huge amount of pre-existing momentum born of years of experience without your presence. And they are winning with their method.
As I reread you inquiry I saw a phrase which always leaves me with despair, a plea for "tips". Just as with anything we do there are no "tips" or tricks or shortcuts to any of our goals, least of all turning the tide of a family. Be patient, educate. It is a much longer course than many families can appreciate yet when you bring this understanding to the table they understand that you are talking about the life of their child, not just some trick and a trophy.Yet this will not happen in one conversation or maybe not even in one season. Be the expert. Share your insight with the family about their child, not just how hard he pedals, and if you are in tune they will know it and you will have their faith. The rest will come.