Inspired by my personal experience, I am going to speak about what I think is the best age to start exercise and this message is mainly directed to parents. In my opinion the answer is quite simple: As soon as possible.
Most people give for granted that if exercise is started at an early age the growth will be affected, that is true to some extent. If a child or even an adult starts lifting weights that the body cannot manage out of nowhere, of course the development will be affected, no matter the age. A gradual introduction to training is crucial to guarantee a healthy growth, better yet a “guided” introduction.
I think it is quite certain that the most delicate time for someone to build inner strength and self esteem is during adolescence, especially at school. School is where many friends are made, but it is also the place where enemies and foes are. It is not unknown that bullying is widespread everywhere and many times it gets teenagers to keep things that happen to themselves, affecting their self esteem and confidence. I know it because that is what I used to do: I grew up in a tough neighbourhood of Sicily and I was very thin and easy to target, I used to stick up for myself without much success, and I would not tell anybody because of my pride. I was lucky because I have always been a proud person, I started exercising and my confidence and self esteem remained high, but I appreciate that not everybody is the same, many people go the opposite way, putting themselves down in sometimes irreparable ways. Here comes the importance of exercise, that it does not really have to be heavy weight lifting. The best option for a young person would be starting a fighting discipline that will help develop that inner strength and confidence for the brain and much more for the body.
I personally have some experience in Boxing and I did want to learn about this discipline to be able to protect myself in certain situations and it most definitely helps me feeling more safe, aware and ready.
So there it is, exercise is not only around weights and it is fundamental for people of all ages. If you are a teenager, help yourself and start training. If you are an adult/parent, help yourself or your kids and start exercise. For a more effective workout and help getting in shape try it with a Personal Trainer.