No prob! I love talking about the more intricate and less known parts of the canon, or the canon that never was, rather :P
As much as it pains me because I find kind of nonsensical, I wouldn't call Astarion being 39 (or, well, 239) a headcanon, as it's what's in the game. I'm not sure if him being that young for an elf is headcanon or not though, since I can't be sure what the intent truly was even if I have my opinions about it. I suppose it's accidentally or not canon since it's set in the dnd universe, so all we can conclude is that he was for some reason that young, as much as we can think it was a mistake. I know some people have tried to explain it by saying that "he lived in a mostly human society, and thus considered an adult in Baldur's Gate" but I still find that flawed both because of the clues from his personality and his looks. Others have said that the trauma or undeath has significantly aged him, which I find clashes with how we conceive most vampires to be, frozen at the time of their death if only looking a bit deader. But people have grown attached to those ideas, and that's okay.
What I prefer thinking is that he's whatever the elf equivalent of human 39 is. (Given that Minthara is ~250? and Halsin is 350, Astarion looks somewhere inbetween that range)
Another interesting thing however is that there is this pseudo-canon mobile game called "Idle Champions" where they established they had worked with his main writer for his bio sheet in that game and there his age at death was established to be 150, IIRC. It was still not old enough in my opinion, but a lot more apt. It was changed to 39 ingame a couple of months later in a silent patch, and I'm not sure if it was because of the writer notifying that mistake, or because of fans noting it, and in the latter case, if the writer was asked for permission to change it. To this day, however, their blog's bio that was in collaboration with his writer has his age as 350, so I haven't got a clue.
Sadly I don't think they'll address it. Perhaps years down the line, in some supplementary book or comic? It'd probably not be from his original authors. Who knows. I also think it'd come with the disadvantage of invalidating some people's ideas (probably invalidate mine, since now they're probably going to have to work around having established what I think is a goof). The advantage of the dates not adding up is that the people who like the tragedy of him dying that young can believe that, while I can think it was a mistake because he looks human 39, not elf 39.
Idk if you've heard of this as well, but Astarion was originally a tiefling, and I find him being an elf much more apt. His tiefling veeery early origins have led some people to believe it influenced his storyline and tombstone, which I disagree with since those aspects of the script would've come much later down the line, when he's already been long established as an elf and not a tiefling.
Unfortunately I have to give credence to the fact that Astarion's personality hardly feels like that of someone who has the lived experience of centuries, unlike someone like Halsin or Minthara. That can be due to the regressing experience of going through all that trauma, I suppose, but when I see him it's hard to think "Oh, this guy was 300 before he died." It's much easier to picture him being literally 39. Which is a problem, given that he's an elf, as little as that's acknowledged.
I also suppose writing centuries-old elves in DND is hard since they're bound to be somewhat human-like because of the universe they're set in, where anyone can play as anything and be whichever way they like, plus conveying the almost alien experience of someone having lived that long is probably difficult, and I think brushed off for most DND elves if possible.
