This would be an opinion. The reason for this is because this is the opinion of a certain individual, rather than it being a fact. It's not a fact that video games are violent for anyone under seventeen years of age because there other games out there that are completely safe for others. There has to be some sort of confirmation of video games actually affecting those age groups, by scientists - which has been proven, but with other games in certain situations.
Others may say their opinion about video games being bad, but others may have a completely different opinion. Someone else may think that video games are actually something very good - and helps one develop in society. This specific statement would be identified as an opinion due to this.