The lack of 10 year olds also really hurts horror films. In the early 2000s, cinemascore published age breakdowns of their scores. Let's grab some 2001/2002 horror films.

https://web.archive.org/web/20021113123120/http://www.cinemascore.com:80/cso2/find_movie.php/Anaconda.html?l=ct&mid=1616

or a Halloween sequel

https://web.archive.org/web/20021226160629/http://www.cinemascore.com:80/cso2/find_movie.php/Halloween:_Resurrection.html?l=ciw

Hannibal (classified as a drama but obviously horror adjacent)

https://web.archive.org/web/20020409154003/http://www.cinemascore.com:80/find_a_movie.epl?id=Hannibal

the ring

https://web.archive.org/web/20021108204747/http://www.cinemascore.com:80/cso2/find_movie.php/The_Ring.html

All of these have at least fine U-21 numbers, terrible 35 and up numbers and mixed 21-35 year old numbers. The problem isn't really teens for horror films its everyone else.

Look at a study with this data to see aggregate age impacts. Unfortunately they don't split out horror to its own section (looking at horror + scifi + fantasy is pretty useless) but general findings are useful.