Parents can't monitor their kids 100% of the time. When I was an underage teen, I got my hands on nudie videos... and my mom and dad had no clue. Same applies to games. This is where the law steps in to act as surrogate parents and ensure underage kids don't get their hands on porn videos, violent game, cigarettes, or alcohol while their parents are away at work.

Surely you are not advocating repealing laws and selling beer and wine and smokes to 14/15/16-year-olds? I hope not, because these items don't belong in the hands of immature juveniles. Neither do porn vids or M-rated violent games.