What is the maximum normal height for an eighteen year old boy?

Depends: It's all in the genetics-there is no solid answer for this. Mathematically you could calculate the "predicted average" for height using his parent's heights (in inches) [mom+dad+5]/2 = average predicted.Following growth curves (done by the pediatrician) over the first 2-4 years of life also helps predict what his final height may be, but there are other factors that don't enter these calculations

Check chart: You can check norms for height/ weight using growth charts. See: https://www.cdc.gov/growthcharts/index.htm