Testosterone is a natural hormone that your body produces. You produce much more Testosterone in your early years because Testosterone is a hormone that helps you develop into an adult by encouraging the overall growth of your body.
As you start going over the age of 20 you produce less and less Testosterone and hence why you don't end up growing into a giant. When your own body produces Testosterone, then your own body monitors this Testosterone production and it is very safe and natural to your body.
Some people who want to grow bigger muscles resort to taking or injecting Testosterone in its synthetic form. This can potentially be dangerous for your body because it is no longer produced within your body and is an outside foreign agent being introduced into your body. This sort of Testosterone can be dangerous for your body and can have many side effects including hair loss, enlarged organs, health issues and aggression.
Another way of increasing Testosterone levels and building more muscle when you are older is to take a Testosterone supplement. A Testosterone supplement encourages your own body to increase Testosterone levels and is a more gentle and natural way of boosting Testosterone levels. When you take a Testosterone supplement as long as you stick to the dosage and you cycle it, like 2 months on and one month off, then you should be fine taking it.
As a whole, many people do take testosterone either in the form of supplements, medicines or steroids. Too much testosterone can be dangerous for health but as long as you are sticking to a testosterone supplement and not overdoing it, you should be ok.