Is strength training good for children and teenagers?
You bet! It is a great idea for kids who want to be active, feel good, and even look good (mostly teenagers). But always supervised.
When can they start?
They can start as early as 7 or 8 years old. The child must be mature enough to follow instructions.
But hold on! Please, do NOT confuse strength training with weightlifting, bodybuilding, or powerlifting. These last three are not good nor safe for children and teenagers. For them, when following a fitness routine, their main goal is NOT to build muscle or lift heavy loads for maximal strength.
It is extremely important to consider that children have young muscles, tendons, and growth plates (those areas of cartilage that haven't yet turned to bone). These could be at risk when lifting heavy loads sacrificing the proper technique.

What if my child is an athlete?
Strength training can benefit the child's performance and reduce the chance of injury when is done properly. āStrength training offers young athletes a lot of benefits, but it must be done correctly and safely,ā says Jeffrey Nepple, MD, Washington University pediatric orthopedic surgeon and sports medicine physician at St. Louis Childrenās Hospital.

Some child athletes are being pushed to add strength training to their program, but they should want to do it. There are some ways to explain to them the benefits of strength training if they really like the sport they're playing. That way, they can want to participate in the program.
How should they do?
While is true that being physically active has benefits for everyone, (physical, and mental to leave the list short), not everyone can follow the same program. Variables like age, goals, medical history, likes/dislikes, etc. must be considered when programming.
For parents:
Here are some tips to consider before putting your kid in a strength training program:
Make sure they work with a professional and Certified Trainer with experience with youth training. A Trainer will safely create an effective strength training program based on your child's age, skills, size, and sports interests.
Emphasize proper technique: They should start light, with their own body weight and progressively add resistance (free weights, bands, etc.) as they evolve and master the form.
They can do strength training 2 to 3 times a week, with at least 24 hours rest in between those days. Never train the same muscle group back-to-back.
They should want to go to the gym and not be forced to. There are MANY ways to make a child/teenager partake in physical activity to promote a healthy lifestyle.
The training must be FUN! This is important, especially around these ages to prevent boredom. A professional would know how to make it fun.
Keep in mind that machines are made for adults' size, thus some of them can be dangerous if the teenager doesnāt fit well.
They need supervision ALL the time: they canāt be left alone for their own safety and the others.