Well the age limits are there for a reason such as one that Len stated. I dont think minors should be out there being able to buy an 18+ hentai manga and/or anime. They give the age limits based on content as it is usually described right on the cover/sticker/etc. The manga restrictions usually tend to be vague and is usually written out for explicit sexual and/or violent content. Same goes for anime, most anime presently have the age limit/recommendation on the back and in most cases they even explain the reason for the rating.
The ratings/limit is there to protect the animation studioes, manga publishers, and even us, the viewers. The creators/publishers need to be sure that explicit content is warned of and recommended to certain age groups as to avoid getting complained/sued by angry parents/etc. Its for the benefit of EVERYONE.