It won't work
Rating games like movies won't work, and here's why:
Most parents think "games are for kids".
Most board games are age rated based on the difficulty of the game rather than it's content. "Ages 8 to adult", for example means a seven year old will have difficulty understanding the game and it's rules, not that it has content inappropriate for a seven year old.
Put the two together and when a parent is being pestered by their 12 year old to buy them a 15 rated game their most likely thought (other than "buying this will shut my kid up") is "my kid is smart for a 12 year old so can handle a game with a 15 year difficulty level.
An entirely new rating system is needed.
Of course, if someone's idea of parenting is to sit the kid in front of the TV/Computer/Games Console so they're occupied and out of the way, no rating system is going to help.