1. As stated by other posters, it depends on the kid.
2. Children are born year round so whenever a child’s b-day is (and school deadlines keep moving so making or missing the cutoff is meaningless), there’s likely to be someone else in the class who is pretty close in age.
3. That said, I actually think it’s better to be one of the younger kids in the class than one of the older ones because the kid can graduate HS earlier and have some breathing room to spend more time in yeshiva or seminary (or even, gasp! college). They can stay for shana bet or make progress toward a degree without worrying about being an old maid/alte bochur.