I feel (and this is likely going to be a controversial take) there's so much to work with regarding Wrathion & his age.
no matter what way you look at it he was completely and utterly robbed of a childhood, but there's also questions about how much his general maturity is "just a dragon thing" vs. how much of it is a result of the experiments performed to create him infusing him with Titanic magic that may have altered his cognitive development vs. how much of it is pure trauma making him "so mature for [his] age!" because his entire ability to be safe and free hinges on his ability to eliminate every and all threats that exist to his safety and freedom. which in his perspective (circa Cata/MoP) was basically Everything because the world is in crisis and so many forces want him, specifically, dead or under heel.
During MoP we see his lack of emotional maturity come through when things aren't going his way, from anything as benign as fussing and complaining about the rules of a game to his absolute raging tantrum when his backing of Garrosh Hellscream didn't go the way he wanted it to. When he reappears in BfA he has developed a fully realized sense of emotional maturity in the interim.
then there's the elephant in the room which is the fact that, regardless of his general level of maturity, numbers-wise he is very young. especially for a dragon. he is literally 12 years old in Dragonflight according to the timeline.
(Cataclysm occurred during years 28 and 29; we'll be generous and count Wrathion's age as starting when his egg was created by Rhea since dragons develop awareness while still in the shell, though he wouldn't've actually hatched until late 28 or early 29 (although Chronicles v. III does apparently declare his genocide of the Black Dragonflight began during 28, it also says 4.3 doesn't occur until 29). Dragonflight starts in year 40.)
In Mists of Pandaria we see he has insecurities related to his age and doesn't like being ribbed about it at all. And, again, he just kind of gets over this off-screen, or else internalizes it to a degree he might as well have gotten over it, because he acts entirely unbothered by Sabellian bringing it up.
There's no conflict with it. There's no grappling with his complete lack of childhood, no negotiating with the narrative that positions him as not being a child nor an adult, no nothing. It feels like his writer(s) in BfA/DF decided his age was an unpleasant feature of his character to be ignored, rather than a core principle of it to be embraced because of how heavily it informs his stances and decision-making — of course he makes shortsighted, violent decisions like genociding the Black Dragonflight, not only was he less than a year conscious but from the second Rhea arrived in the Badlands to create him his existence had been consumed and defined by gruesome violence and the stripping of autonomy, whether his own or Nyxondra's or even the egg sacrificed to Deathwing in his stead (which probably didn't consent to that.)
Wrathion's actions in Cataclysm and Mists are vitally contextualized by the understanding that he has no personal awareness of times pre-Cataclysm, and that while he can learn about those times through research, the fact is that for as long as he's been alive it's just been war.
but we're not going to focus overly long on all that stuff, it's Awkward and Ugly and Deeply Uncomfortable, and Wrathion is our Cool Witty Rogue Guy! let's just brush all this uncomfortable stuff under the rug. we'll pay some of it a tiny bit of lip service so they can't accuse us of not acknowledging it, even though we are effectively not acknowledging it, and move on.
