Weight-lifting and running at a young age is a hot topic!
I had a young patient (7 years old) whose mom was concerned because he wanted to start lifting weights...their neighbor was a body-builder and, of course, this caught the young boy’s attention - he wanted to be “buff” too.
Mom’s concerns regarding “damage to the growth plates” and “stunting of growth” are definitely valid, but there is a middle ground.
At the risk of being overly technical, it is important to understand the process by which bones are formed and continue to grow.
Mesenchymal cells come together and differentiate into chondroblasts (“chondro”: cartilage, “blasts”: builders). The chondroblasts form a cartilage model of the bone that has an outer layer of perichondrium which turns into osteogenic (“osteo”: bone, “genic”: producing) cells. The osteogenic cells become osteoblasts (“osteo”: bone, “blasts”: builders) which form a bone collar around the cartilage model. The primary ossification center forms in the center and the cartilage model is transformed to bone. What remains is the secondary ossification center (aka the growth plate or epiphyseal growth plate) at each end where the osteoblasts continue to form bone in order to increase the length of the long bones.
Simply, cartilage forms a soft mold of the bone which eventually begins to harden from the center leaving the two growth plates at each end. It is from these growth plates that the bone continues to grow in length.
The epiphyseal growth plate is very much alive and therefore somewhat fragile. There is a perfect balance between stress and strain that promotes strength and positive adaptation versus overuse and overload leading to damage and breakdown.
The only true way to know when growth is complete is through X-ray. Generally, this occurs around the mid to late twenties.
Strength training and weightlifting is definitely beneficial as proper form and movement mechanics can be learned at a young age. This is actually ideal! Young athletes should focus on coordination and body awareness to not only prevent injuries but also to maximize strength gains in the future. When their body is mature enough, they will already have the foundation to build on.
Kids and youth should be having fun (most importantly!) and experiencing variety (variety in sports, variety in training programs). These early developmental years are the opportune time to maximize on a variety of athletic endeavors and experiences - the more exposure they get the more well-rounded they will be. They will never be more adaptable or learn more quickly than they will at younger ages.
There are some awesome pieces of equipment out there for kids and youth. Light resistance and high reps protects the growth plates while enhancing coordination and perfecting movement patterns. This alone will increase strength as the neuromuscular system becomes more efficient.