As the kids have gotten older, the content of these Harry Potter films has also gotten more mature. Sure, there's always been some scary stuff, but now the kids are dealing with a ton of fighting and violence…and, yeah, even death.
We've seen that before, of course—after all, we lost both Dumbledore and Sirius in previous films. However, this film is exceptionally heavy on violence and death, with lots of Hogwarts folks losing life and limb in the Battle of Hogwarts.
It's scary, suspenseful, and could definitely be scarring for the under-13 crowd.