While 18+ is obviously adult only, I wouldn't equate it to the esrb adult only. AO esrb games would be something intensely violent like uncensored Man Hunt 2 or something erotic in nature, whereas I'd say Cero Z is more like a slightly harder M in the same way you have Pegi 16 and 18.
12 Year Olds - Twilight Princess HD, Genei Ibun Roku #FE, Project X Zone 2
15 Year Olds - Fire Emblem If, Xenoblade Chronicles X, Bravely Default
17 Year Olds - Devil's Third, Bayonetta 2, Attack on Titan
18 Year Olds - Zombie U, GTA, Skyrim