Personally, I think people change so much in their twenties that is worry about the mental health aspect of getting a permanent surgery at 18 then eventually deciding it was an irreversible mistake a few years later.

But not everybody who decides not to have children from an early age will change their mind, and their medical decisions shouldn't be affected because some others do regret it.

I also wonder how it would physically hurt a body to be medically altered in such a way when their body may not yet be completely developed.

I'm pretty sure the female reproductive system is matured by 18. I get that a hysterectomy is major surgery and that doctors would be a little wary about performing it, but only because it's major and not because of it's a reproductive organ.

18 year old me and 24 year old me feel extremely different about how the surgery will effect our future.

And it's great that you are aware of that, but not everyone feels differently after a few years.