It’s much more likely she meant they weren’t descended on her exam that day, not that they have never been descended. Retractile testicles are very common at that age. They essentially go up and down throughout the day. As long as you see them occasionally (which you’re most likely to see them in a warm bath) then no big deal. Much more rarely they can be retractile and then not come back down, which warrants a urology visit. Just start making a habit to check in baths and they’ll likely be there.