Sounds more like you simply read those kind of stories.
90% of all the series I have read the younger character is indeed over 18 years old or the older character acts like a parent/older sibling and not in any sexual manner.
"90% of all the series I have read"
Exactly.
I have a very wide breathe of genres I've read due to the many years of manga/anime I've been exposed to and various friends who recommended various things. I can say with absolute confidence that.
A - The vast majority of characters in manga are the ages I listed before.
B - The vast majority of characters in manga with fanservice are the ages I've listed before.
C - There are a ton of younger to older person relationships in manga involving characters under 18, just most of them go under the radar. You wouldn't believe how little people care when the MC is a high school boy in a romantic relationship with a college aged girl or older.
D - People see what they want. Tons of "parent/older sibling" relationships in their heads that were in actuality written by the author to be romantic/sexual at it's core.