None of the objectivists have addressed this:
Why should you be entitled to buy M-rated games? The store would gladly sell them to you to turn a profit, and to everyone else, but that angers parents and causes them to take their business elsewhere. So, it is in the store's best interest to regulate who can buy what so that adults - who plop down thousands for TVs, stereo equipment, etc. - will be more willing to shop there for their family.
And that is one of the main reasons. Not 'morals', but because keeping the big spenders happy is the rational thing to do. And besides, if you're a 'man', you won't be held back by the edicts of society! You'll do what you want, because you are a free, rational individual!
...which is why you thus obey the laws that society has set forth for everyone's benefit.