I dunno... in a lot of ways, the young mind is more receptive to learning than the older mind. Children can learn some pretty complex things, like languages. A child who's taught a second language from an early age can master it far more easily than an adult who's never studied it before, because a child's mind is more plastic, more adaptable. In a way, higher mathematics is just another language. Maybe if you fold some calculus ideas into a child's mathematical education from early on, treat all of mathematics as a single discipline rather than arbitrarily dividing it into separate courses, it could turn out to be easier to master in the long run.
I dunno, maybe that's a stretch. Another possibility is that what we saw in those TNG episodes was not typical. After all, the personnel aboard the Enterprise
-D, whether Starfleet or civilian scientists, would've been the best and the brightest, a ship full of geniuses. So it's plausible that their children would be budding geniuses as well and might be in more advanced educational programs than the rank and file.