Although this is essentially up to the parents to judge, I simply don't think that ignoring age ratings on games is harmless. Children can be very impressionable and might not be able to take video game content with just "a pinch of salt". Simply ignoring age ratings can result in young children being exposed to all the unthinkable graphic violence, nudity, rape etc which exists in many games today. Especially as games are getting more and more realistic. Yes it is up to the parents and how they deem their child but simply ignoring age ratings all together is an ignorant and potentially harmful step.
Playing violent video games doesn't make you into a psycho or a murderer, there a safe alternative to getting anger out without causing physical harm. The games are fun and cooperative and most include a setting that removes blood and gore/language. They greatly improve hand/eye coordination as well as strengthening the mind
I've been playing violent video games since I was 8 and I've never attempted to reenact any of the actions depicted on a video game or on a movie, That goes for other kids I've grown up with as well and kids I know today. It also probably good for them because studies show that there's been a decrease in youth crimes since the release of video games. Beside, like Steven says it also limits stress.
Most companies over rate their games. E 10+ games can be played by children younger than 8 years old. Most games are just slightly violent, e.g. a wrestling game or something similar. Younger kids are fine playing them, but some parents are overprotective and don't let them play games that are perfectly fine.