The answer depends on the background and training. My son as an 8th grader ran 35:23 for 10k on the roads and 17:08 at Woodward Park (site of Calif state xc meet) later that fall. But he was reasonably trained, having run 25- 35 miles a week during middle school. If he had been an occasionaly jogger, the 39 minute mark would have been good.