Whenever both recipents (parent and child) feel comfortable about discussing and agreeing about the more mature content in games, and by extension, anything with violence/swearing/etc. really.
I feel as if nowadays lazy parent can't be bother to monitor and be interesting in their own child's interests; they just throw them an iPad/game and let them be because they can't be bother to engage with them on a personal level.
For instance, my sister (who's recently went to work at a primary school) showed me a video of a FIFA youtuber that her younger students watch on a regular basis/subscribed to (basically his entire fan base). This guy, who is pretty popular amongst the Youtube community, was furiously shouting swear words all of the place, making fun of kids that were no less than 10 years old that sent him videos, and even at one point talking about having sex and masturbating to dead bodies.
But this wasn't even shocking to me. Kids can easily sign up to Youtube, lie about their age (I think you "have" to be 13 to sign up to Youtube), and be open to this kind of content.
The same can easily be applied to games. If I had a son who wanted to play GTA 6/7/8/etc., or wanted to watch a higher rated film, or even a book, I wouldn't take the chance and let it slide, giving in, I would engage with his pretty obvious interest in the game, take a look at it, and see whether or not it is suitable, and being "suitable" is up to your own personal opinion.