If you're a guy and you've started puberty, your body will have begun making the hormones necessary to help build muscle in response to weight training. If you haven't started puberty, or you're a girl, you'll still be able to get stronger - you just won't see your muscles getting much bigger. Before you start strength training, you should be checked out by your doctor to make sure it's safe for you to lift weights. And to reduce the risk of injury,...