The erection may occur without stimulation. Your child's penis may be dark red or purplish. Priapism is usually painful and can lead to permanent tissue damage. Erect lengths average from 12 to 16 centimeters ( to inches). Erect . Men were excluded from the study if they were under 18 years old, presented with a. Find out about boys' bodies, including puberty and sex, penis size, erections, Most boys start to go through puberty when they're around 13 or 14 years old.