Why is Hentai Censored in Japan?

I remember reading something about it months ago, but never fully understood.
Who is it censored for? Will there ever be a day where this might change?

Cocks aren't the only thing censored : vulvas too.
And I'm pretty sure you aren't afraid of vulvas.
Vulvas are great.

I think hentais are censored for 16-18s and uncensored for 18+. They are strictly sold as OVAs and there are two versions.
I think they give you the "appropriate" version depending on your age when you buy it.