An arbitrary at best rating system isn't likely to stop them from playing any other M/AO game either, because it simply hasn't to this point. The entirety of rating systems work like this, Parental Advisory albums, R rated movies...On and on and on. It isn't even shops doing it, most of the kids just have parents get it for them, or friend's parents, and then if all else fails try it and see.
It's more of a lack of enforcement. Both the MPAA and ESRB rating systems are not legally enforced. They are voluntarily enforced at the store level and usually these restrictions come with it (I think most stores have similar policies for Rated R and NC-17 media respectively)
M-Rated: You get carded when buying (refused if under 17). Parent allowed to buy for child.
AO-Rated: Nearly all stores will refuse to sell them. Must be 18 years to buy and will be carded. Parent not allow to buy for a child.
Even then, it's up to the person running the cashier to enforce it. Most retail stores care more for profits than they do for their policy and will only make a big stink if not following policy nets them bad PR.
There will always be 13 year olds playing games they shouldn't, and popularizing a new, more deviant rating isn't going to provide another barrier to entry for these kids. They are going to find ways to play the AO games, which if past AO games are any indication, will feature themes and motifs that are objectively horrible and lacking in any redeemable social benefit (see: Hatred Link).
If you popularize the AO rating, it's not going to become the replacement for M rated games. It will become a new standard, and impressionable people with still forming brains might miss some attempt at satire and take games like Hatred seriously.
Also it's not unreasonable to think that games would be skewed more towards low hanging fruit like sex and gore and violence than before. Sacrificing story, art style, and creative direction in order to turn profits by abusing young people's fascination with the taboo. It's already happened with other media, I would hate to see video games follow suit any more than they already have.
Btw, Fajitas > All.
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