Physical appearance is important to adolescents, so it's natural for teens and pre-teens to want to start working out. Skinny beanpoles or pudgy babies who grew into pudgy kids may start having visions of building muscles to re-shape their bodies. Before puberty, however, kids' bodies aren't ready or able to build significant muscle mass, so if they attempt to gain muscle before their bodies are ready, they are likely to be disappointed.
The Necessary Ingredients
It's not just lifting weights and performing other resistance training exercises that develop muscles. Two vital hormones are necessary to gain muscle, and the body doesn't produce enough of them before puberty to build significant muscle mass. Without increased production of growth hormone and testosterone, resistance training won't result in much muscle gain, if any at all, for children. That's why kids under the age of 15 don't typically look like Mr. Universe or a fitness model.
When Muscle Gain Begins
The fact that puberty -- and therefore increased testosterone and growth hormone production -- doesn't begin at the same age for everyone can cause frustration for a teen who is impatient to start gaining muscle. Some kids hit puberty as early as 9 years old while others don't start until as late as 15. Between those ages, kids can start looking for signs, such as growth spurts and acne. They are fairly reliable indications that puberty has begun, and muscle gain is possible. Since testosterone causes facial and body hair growth, that's typically a dependable signal that a person has the right hormones to start building a muscular physique.
Working Out Before Puberty
Just because children can't gain significant amounts of muscle before puberty, it doesn't mean it's a waste of time to work out. Getting started early in physical fitness is a useful habit to develop, one that kids can carry into adulthood. Weightlifting before puberty used to be discouraged, as it was thought that the child could be harmed or his growth inhibited by placing heavy resistance and stress on growing bones. But if pre-pubescent children lift weights under the supervision of an experienced instructor, they shouldn't experience adverse effects. Kids can safely perform bodyweight exercises, such as chin-ups, sit-ups and push-ups, and engage in resistance training with light weights, ones that they can lift with no trouble for about eight reps.
Optimal Muscle-Building Age
The window of time for optimal muscle building starts closing as soon as it opens. It's about a 10-year period, so a person has a good number of years to build up muscle between the ages of 15 and 25. After that, it gets more difficult for both men and women to gain muscle mass. And if you don't use it, you'll lose it. Without continued resistance training, people start losing muscle mass after age 25.