1.5 years isn't even that much? I mean you can probably find that variance within any cohort so I doubt it was the acceleration that caused any psychological repercussions.
Besides, my experience is that if you’re so bright, there are plenty of opportunities everywhere to satisfy your curiosity (or at least work on it). It does not have to be at school.
That said, girls tend to mature faster than boys, and I can assure you that most 18 year old females are not very interested romantically in younger males (in that era and place, at least). A year makes a huge difference in that regard. I suspect the study in question didn't account for this.