Fair. I just know my first character was a Human Paladin, who was probably mid-20s. Based on what we’ve seen, it seems likely that canonically, that same character would now be 40s or even 50.
I was still in my teens when WoW launched so the character would’ve still been older than myself, but the passage of time just creates a weird sensation that “maybe this timeline doesn’t actually work anymore”, because your player character would obviously be well past their “prime”.
It was really never an issue, until Dragonflight. That’s when they decided to implement a time-skip, and now we’ve got a much older Anduin and Dagran. Even Faerin seems like she would’ve been an infant or a child when Vanilla launched, and now these characters are leading the story. Just feels odd.
For Denona, she was a member of the Kul Tiras Elite Corps that occupied Jaina to Kalimdor.
I believe Faerin is a little older than Anduin, I think she mentions having been in her teens when she stowed away. That would put her in her mid-to-late 20s now.
There’s always been some startlingly young characters running around. Obviously Anduin has grown up with the game, but Talanji’s about the same age as him and was therefore in her late teens in BfA. Taelia Fordragon’s age is vague, but she seems to also be about the same age as Anduin. Valeera Sanguinar was a child when the Scourge invaded, making her a teenager in the Vanilla timeframe when the bulk of her comics are set. Vanessa VanCleef was a teenager when we first fought her in Cata. Lilian Voss grew up in the Scarlet Crusade, meaning that she was at most in her early twenties when we met her in Cata and quite possibly was still in her late teens. Darion Mograine was at most twenty when he first died and was likely younger, meaning that he was in his early twenties when he founded the Ebon Blade. Sally Whitemane’s only a few years older than Darion, having been a teenager when Lordaeron fell, and she and Renault would have been in their early 20s in Vanilla. At least with the time skip, all of these existing young characters have added a few more years to their ages and seem a bit more reasonably aged for their occupations. Dagran II is still very young, but he’s also usually paired off with an older character whenever he’s doing something so that he’s not running off by himself.
We are still a little behind how much RL time has past. Remember, some of the expacs that we played for 2 years were considered only one year in game time.
Our best timeline looks to be: WoW release was canonically 25 years after the dark portal. TWW is ~41 years after. So, in game it has been about 16 years to the 20 RL years. I am betting we get another time skip after the World Soul Saga that gets us closer to RL time passage.
Best info puts her a bit older than Anduin, so more likely a teenager in Vanilla.
I think that is just because in the RL we have gotten so used to all the political leaders being as old as dirt. 
My character has whatever age i currently have irl so i retcon whatever is needed ro keep it that way. It is a self insert power fantasy after all. If we are on year 42, then Barth was 15 during cata when Gilneas fell.
Then don’t. Classic characters never really age.