In Australia most kids do not live away from home when they go to University unless the travel time is rather large.
The issue is the degree of content.
An R18 rating is a much stronger message to parents than an M15+ title, a parent willing to jump one category because they decide their child is can cope, may not decide to go with an R18 rating.
As things stand however, every single game in Australia has to be acceptable for a 15 year old to view. Games that based on ratings for other places are not are making it through with an M15 rating. This is not acceptable for various parental groups. That games also do not make it through uncensored is unacceptable for adult gamers. Clearly a solution is to add the rating.
Another solution would be to make the rating system optional, which also would allow games where getting them rated is impractical (such as downloadable games, especially via the iTunes store) to have an official position rather than just a "We haven't yet decided to enforce it" stance that is currently in effect. I don't see this happening any time soon however.