I do agree that a 20+ year age difference with the younger party being in their late teens/early 20's is a bit creepy.
As others have elucidated to, it's usually a power/control thing, with the older partner seeking out someone more malleable who they can shape to be the partner they'd like.
Your late teens & 20's are for growing emotionally - experiencing different relationships, being single, learning independence, having fun, making mistakes. Perhaps travelling, having a career, living elsewhere.
Sticking with 1 relationship at such a young age limits you in life.
It's fine having a 20 year age gap in your 40's & above, when you both have life experience.
But at 20, it's a case of "you don't know what you don't know".
I'd be horrified if my 20-year old DD was dating a man in his mid-40's. And I'd feel pretty yukky about any of our now-mid 40's circle of friends dated a 20 tread old.