I have put together a good list of training methods for younger people who want to increase strength or put on a little muscle.
Before we get started, let me say that weight lifting will not stunt your growth. Click the link for studies that have researched the theory. In contradiction, improper eating and inactivity is much more likely to get in the way of healthy growth and development. Exercise and physical exertion is a natural occurrence that the body was made for.
Over training and improper form can cause injury. This is true, regardless of age. For this reason, it is extremely important that you learn the right form and do the exercises properly when you start strength training. Failing to do so can hurt you.
Exercises according to age
Exercises for children 9 years and younger
Children who are aged 9 years or younger need to focus on developing the right motor skills. This improves the mind to body connection. The best way to do this is through movement. Make these exercise sessions light and fun. Try doing a variety of activities like catching or throwing a ball, chasing or running away from someone and moving around. This improves spacial perception (correct body placement in accordance with a task and the environment). Stretching and trying out new movements is also good. Keep exercise sessions relatively short, unstructured, and fun. Also make sure that these sessions are at low intensity levels and don't push the child to the point where they start to perceive exercise as a negative endeavor.
Strength training that involves weights is not suitable for this age group. They do not have the technical ability to perform this kind of training with the best form, which could very easily lead to injury. Focus on fun activities that will improve a large variety of skills that can be implemented into almost any sport.
Strength training for children ages 10 to 13 years
At this age, people can start training with weights. Free weights are better suited, since they closely resemble natural movements that are often used in sports. Light weights and high repetitions are the word of the day here. Keep the repetition range per set at higher levels. Do not do sets where you cannot do less than 12 reps without resting.
Focus on using perfect from: Doing the exercise exactly the way they are supposed to be done. Using the wrong form can cause injury and learning the right way to lift weights is crucial for the rest of your life. Train using weights for a maximum of 3 times per week. Give your body time to adapt from the pressure that you put on it from training. You can do sports and cardio exercises on other days. Body weight exercises aren't recommended because the intensity is too high. For example, if you can only do 10 push ups, there are not enough repetitions in the set for it to be the best sort of exercise for your body right now.
Don't do compound or complex movements either. These are exercises that work more than one muscle. They are extremely technical and are very easy to get wrong. Dead lifts and weighted squats, for example, can hurt your back or knees. Focus on isolate movements that work only one muscle at a time. A big focus is to see this as strength straining instead of weight training. Focus on increasing your strength and do the exercise that you are currently good at doing according to your strength. Focusing on weight training will make you try to increase weights too quickly and this can go very wrong, very fast. At this stage, do all your weight training exercises in one workout session for best results. 1-2 exercises per muscle group works best.
Strength training for children ages 13 to 19 years
Now you can focus on weight training with a little higher intensity. You can decrease your rep range to 10. This means that you can increase the weight so that you can only do 10 repetitions. If you can't do 10 in one go, the weight is too heavy. You can do strength training with weights up to five times a week. If you are experienced with strength training, you can split up your training program so that one day focuses on one muscle group while another day focuses on a different one. For example, Mondays can be leg days and Tuesdays can be back days.
You can do body weight exercises, but make sure that you allow for at least 48 hours (2 days) of rest between the times that you work the same muscles. If your body is comfortable with weight training, you can start using compound exercises. Be careful with these: Make sure that you do them properly and use the right form. Using the wrong form can hurt your body more than benefit it. If you get injured, your body will get weaker than when it started. This is why you need the right form and enough rest. You can find good workout plans for beginners (13 years or older) here. Do the first one very well for a few months before moving onto the second one. With the second one, make sure that the lowest rep range you use is 10 (instead of 8 as some of the exercises say).
Focus on good form and give yourself enough rest. Eat a well-balanced diet that includes lots of fresh fruit and veg, meat and dairy and carbohydrates. STAY STRONG!