The Centers for Disease Control growth charts indicate that 50 percent of all 13 year old boys range between 88 lbs. and 115 lbs. About 100 lbs. is at the 50th percentile for boys this age.
At 13 boys are on the cusp of major growth spurts, and may have already had a growth spurt that help them mature into fully grown men. As such, their weight and height can vary wildly. Most boys can range in weight from 75 lbs. up to 142 lbs. at 13 years of age, as stated by CDC growth charts. About 50 percent of boys are at or below 100 lbs. at this age.
There is a formula that may be useful in predicting the final height of a child based on the heights of both parents, but there are often exceptions, and it is considered more of a guideline than a hard and fast rule. Add 5 inches to the mother's height, and the result is added to the father's height in inches before being divided by 2 or (mother's height + 5 inches) + father's height divided by two.