To answer your question, no, there isn't an age limit. The team is expecting way more 18+ members that kids, but it is really up to the parents to decide. Keep in mind that this game contains no nudity or inappropriate content in that matter, and the only thing to fear in making the decision is the optional drugs, guns, fights, cursing (by human players), and possible in-game death.
In my personal opinion, I don't think there should be an age rating. Identity should be for everyone, including some children, except for the annoying ones (they would be weeded out by moderation). My little brother doesn't even use the mic whilst playing DarkRP, afraid of facing persecution and not being treated like a normal player, even though he isn't a stereotypical "squeaker". He also enjoys Arma 3, too.