Film Ratings aren't worth looking at, either. The ratings have relaxed so much and are so inconsistent over the years that identical films spanning decades can have wildly different ratings. As Splodge said, the 12A rating is the go-to replacement for PG and U it seems now, as most films aim for that.
I'm just saying, we coddle our children. We try and remove all the horribleness from nature, yet stuff them full of hamburgers while consistently lying about where they come from and how they're made. At what age should you start teaching kids about that aspect of nature? I dunno, it depends on the kid. I know younger kids that are really smart, and I know older kids that probably aren't ready for it.