Minimum of 35 years, like the president.
Or if we really want mature players, I say we set the minimum to 50 years old.
I kid, sorry Butch, you know I love you, but the age limit is not the problem. While being young definitely makes it a bit harder for various reasons, it'd be silly to deny people simply on their age. I've staffed in communities on various games for as long as I can remember, starting at least at 10 years old. If it proves a point, at least as far as I could tell and/or was told, I still made a great staff member when I was far younger than 18. Sure, you're not gonna be perfect (not that anybody is) and immaturity may definitely present itself, but setting a hard cap at 18 would be ridiculous. For a game like Arma, sure, but not for a game like DarkRP where the average age of players and applicants couldn't be above 15-17. Plus, am sure you've seen children who're a lot more mature than many of the adults on the server, it's just a bad path to go down where you preemptively judge people based off of their age.
You're right in terms of inconsistencies, but that's not relevant to age, the inconsistencies are fault of a weak motd, poor training, and lackadaisical staff management. I've spoken my piece on all of that already. With my experience in staff management, I'd recommend what many communities do, and what I even employ when it comes to managing my own bases. Younger people should be vetted a bit harder to make sure they can handle the stress and complexity of the job. You're right in terms of immature staff being a problem, but that's immature staff, not young ones. Many of the best staff in this community are below the age you recommended. I think extra supervision, training, vetting, and assistance should definitely be used, but not a flat out cap.