What has been suggested, aside from the fake ID, is not illegal. I spent more than 1o years in the theater industry, starting as a concessionist and worked my way up to management. The rating system is a guideline suggested by the MPAA (Motion Picture Association of America). It is up to the individual theaters/theater chains to decide whether or not to enforce the ratings. Personally, I think that the ratings are mostly arbitrary, and based on the lowest common denominator in the public. Essentially, the rating system places the movie theater in the role of a babysitter. They assume a maturity level of the general public and decide whether someone who is under 17 is able to view sexual content and language. And then rely on the theater to enforce the ratings. In Europe, the ratings are different, taking into account the maturity and social values of society.
A few things stick in my mind years after I've left the industry, as I enforced the ratings. Parents attempting to bring toddlers into Hellraiser III, at a late night Friday show! This movie was rated NC-17 for graphic violence. It boggles the mind to think that the parents felt it was okay. I told them no, they were not allowed to bring a toddler into the movie. I'm definitely proud of that moment. And second, I was working the afternoon shift as a cashier on a quiet summer afternoon, and saw a teenager walking across the lot to the theater. The parking lot and driveway was at least 3/4 of a mile long, so this kid was definitely making a hike to see a movie. He came in, and was probably about 15. He wanted a ticket to an "R" rated flick, I don't remember what it was, probably something along the lines of Lethal Weapon. Nothing too sexual, but relatively violent. When I told him "no, he wasn't 17" he didn't put up much of an argument, just turned away and walked home. I was doing my job and enforcing the ratings, but really, what harm would it have caused to sell him a ticket. He looked mature enough, and this was proven to me by his acceptance of the "rules." I learned that day to make exceptions to the rules.