It's never too late to start a fitness program that adds muscular size to your physique. However, as you age, your levels of testosterone, the muscle promoting hormone, decline. That's not to say that you can't add muscular mass after the age of 30. Many people have. It will, however, require a conscientious effort and hard work over an extended period of time. It won't happen over night, assuming you train drug-free. You'll need to make sure to pay attention to obtaining proper nutrition and rest. Each of us react differently to training stress, so, you'll need to see what works for you.
Starting a fitness program has other added benefits that include weight control, lower risk of disease, and improved quality of life. Don't let the desire for muscular mass be the only reason for starting to exercise. Set small goals. Be consistent, and, train hard.