After reading this, I can personally say that I used to have night terrors. When I was younger, my dad would tell me of the many nights that he would wake up I would be standing there, in front of him, looking extremely distraught and making weird noises. He would then have to get up and lead me back into bed, where I would sleep for the rest of the night. When he would tell me what happened the next day, I would have no recollection. Eventually, these have gone away, as you stated, and I have not had one of these episodes in a long time.
That said, this article claims that night terrors are mainly found in boys ages 5-7, which coincides with when I had my episodes. I found it quite unusual that your 17-year-old cousin would suffer from these, even though it was still entirely possible. Looking into the subject, it was interesting to see documented night terrors on film. I found it odd that through all of the apparent distress the subjects are going through, they usually don't remember it, and seem to not even know they do it. In the case of children, it is exponentially more common for the parents seeing it happen to be more scared than the child experiencing it.
It was also interesting to find out the reason behind children suffering from night terrors more than adults. This is due to the fact that children spend more time in non-REM sleep than adults, and the fact that night terrors occur in non-REM sleep. It was also interesting to see that nightmares occur in REM sleep, so there is really no link between nightmares and night terrors.