Is 140 lbs a good weight for a 19 year old?

There is not nearly enough information here to answer your question. Is the person a male or female? How tall are they? What is their build like (small, medium, large).

There are many BMI/weight calculators online, but as a serious fitness junkie I have found this one to be one of the best because it asks the right questions and is very accurate. It will tell you your ideal weight. http://www.active.com/fitness/calculators/weight/