I knew you'd post a link to TV Tropes.

But considering the possibilities, it could be because of the chromosomes.
I mean, if that is the theme of X and Y, then either there are going to be a lot of large words that one would require a dictionary to know what it means (especially since I doubt school children will learn about chromosomes pre-Grade 10 - then again, I dropped out of school in March 2011, near the beginning of ninth grade, so I wouldn't know; and I don't even know if this applies overseas considering that things to teach are way different overseas) or something.

As far as I know, just about all TV programs (cartoons) get a PG age restriction instead of A for All nowadays, in South Africa.

...

Merely stating my opinion, of course.
If anything seems offensive, sorry, I don't mean offense. (Kinda hard to concentrate while having a broken left arm.)