They really only say 13+ because there might be some mature stuff. Like Doki Doki Literature Club. That’s 13+, but I found out about it when I was 11. But I wasn’t scarred for life, because I knew about the stuff.
(I put the next things in white text because it’s 13+)
Examples: A character hanging themself, a character stabbing themself, stuff like that.
Which is why movies become PG-13, because 13 is the certain age of maturity where they can see mature things and not be confused. Obviously, kids mature at different levels. So really, the 13+ thing is just saying ‘you must be mature enough to see stuff that is supposed to be 13+’.
Sorry for the rant. Whoops.