How big should a penis be in a 13-15 years old?

It varies by individual. In general, the boy’s boner at age 13, which is usually when puberty has just started, should be at around 4.5 to 5 inches, measured along the top of his hard shaft. By age 15, due to hormones and frequent erections, the boy’s boner should grow to nearly 6 inches when fully aroused (precum flowing).