This goes in the same category with people saying playing video games is for children. In my experience (and I'm speaking strictly from my own experience and not trying to pass these as facts) those are:
1. Old generation people (in their 40's-60s') who haven't caught up with the times, which is normal (people rarely change their music taste past mid 30's for example or inclined to learn / embrace new things).
2. Insecure people who are concerned with being "cool" i.e. not being "a geek" (very outdated view nowadays and hence even more weird one to have) and / or being "adult". Those are often quite immature on many levels while trying to be this mythical "adult" entity or are simply ignorant on the subject (sometimes can be "educated", but rarely have desire to be).
Both of those often are also the ones to whom it is so damn hard to grasp that age restriction on games are there for a reason. Games being for kids and all... then complaining about violent games and whatnot.
Then there are normal people who simply don't like video games. Same as for example me not liking football, rally, hip-hop and whatnot. Those don't claim / throw terms like "childish", "for kids" etc., they're simply not interested in said activity / hobby whatnot.
More or less same here. I for example disliked the art-style at first for various / unknown reasons before it grew on me. "Childish / for kids" was not one of the reasons. I fail to see how game as complicated as V (if not more) can be considered "childish / for kids" whatever the art-style. Now if the game went and devolved into Revolution style "deep" gameplay while having photorealistic graphics... I WOULD call it "childish".