A very common question that arises when deciding to do strength training is at what age to begin this journey, I spoke to Dr. Deepak Gandhi, a pediatrician and a level 3 fitness trainer in the UAE, who told me that strength training starts from childhood, as any activity that strengthens your muscles can be classified as strength training. Activities like playing basketball, swimming, jogging, etc. all help condition different muscles making them stronger. 
However, in routine gym terms, we use strength training for specific types of exercise programs where there is a focus on building strength and not necessarily muscle mass.
Dr. Gandhi recommends that for lifting weights, children start at the ages of 12 or 13 under adult supervision. At this age, the aim of weightlifting must not be to bulk up and become a strongman; instead, it should be to maintain a healthy body.