In all my teen wolf fics, I ALWAYS make Derek 19 in s1. There are multiple reasons why. One, I can't justify his weird antisocial stalker behavior otherwise, two, it makes more sense narratively, and three, I don't feel comfortable with Sterek if there's a larger age gap.
A 19 year old Derek being fucking scared out of his mind and struggling with the loss of his sister, but desperately trying to play it cool and aloof while having no idea how to actually talk to people is something I can completely understand. Like, yes, he's absolutely a creepy stalker at times, but he genuinely has good intentions, (and, in my fics, he's a stupid teenager too) He can be rude and insulting, and threatens Stiles and Scott multiple times, but that doesn't change the fact he doesn't want Stiles and Scott to actually get hurt. Derek's still actively protecting them, despite all the honestly kinda terrible things they say and do to him (mostly s1, it's a shit show, so I don't hold it against any of them too much, and I don't blame Stiles and Scott for being harsh at times, they were scared and didn't know Derek at all, I'd be scared too)
I think Derek's a little less mature due to his trauma, honestly. For starters, he was hiding in new York for 5-6 years with just his sister, and it seems like that's the extent of the social interaction he got. He never really had to learn how to talk to humans, since he just had his family of werewolves, who are experts at reading people and especially each other (being able to smell feelings and all) so yeah, of course Derek has no clue how to actually talk to Stiles and Scott. Secondly, he blames himself for the fire (and pretty much everything else, from what I remember) that would absolutely fuck you up, thinking you're responsible for your whole family's deaths. His anchor is literally anger, he's constantly punishing himself. Like, no wonder he's such an asshole. Thirdly, he has no trust in humans whatsoever (and I don't blame him) so yeah, he gave Stiles and Scott extremely vague answers and wouldn't elaborate on anything, because the last time he was open with a human, his entire family died. It's obvious in the way he talks to Scott about Allison. Like, when Scott says, "She doesn't know anything!" Derek replies, "What if she does?" And then later on, Derek yells at him, saying, "You're 16! You're not in love!"
I didn't even notice it till my most recent rewatch. He's so harsh about it because he lost everything to an Argent, because he thought he was in love, and doesn't want the same to happen to Scott (and Derek believes that humans, especially hunters, aren't safe. He doesn't want Scott to get hurt, and doesn't want to get hurt again himself)
And, alternatively, I think Stiles is more mature due to his trauma. Despite losing everything, Derek could still rely on his sister, while Stiles didn't have anyone, really. From what I remember, it really sounded like he was the one trying to take care of his dad, who was usually busy, and when he wasn't, had a drinking problem. He had to grow up fast, and yes, I know he doesn't always act like that, but we all know he just uses humor as a coping mechanism, and to prevent people from seeing how he's actually feeling.
He's dramatic and sarcastic, but he's also the one who immediately accepts Scott's a werewolf and spends all night researching it, he takes care of Scott and sticks by his best friend even when Scott tries to murder him, the few times he lost control. He tries to take care of everyone, honestly, and sure, he's an asshole about it, sometimes, but he's still there. He might be bitching about it the whole time, but, for example, in Magic Bullet(S1ep4) despite barely knowing Derek and being outright scared of him, Stiles never abandoned him.
Plus, at the end of the season, Stiles defends Derek! He snaps at Chris, who is threatening him verbally and physically. Like, he so clearly allies himself with werewolves as whole, not just Scott, even when ppl are threatening him, and he continues to do so, he never ONCE throws Derek under the bus (after he corroborates Scott's lie about Derek being the murderer) and like, that's kind of everything. Especially to someone who lost everything at the hands of an untrustworthy human, I imagine.
And, I mean, both Stiles and Derek make threats, but they rarely actually mean it. Stiles had no intentions of dumping Derek on the side of the road, and Derek had no intentions of actually killing Stiles or Scott.
The fact that Derek still tries to help Scott, after Scott accuses him of murdering multiple people and puts him on the run again, is proof of that, in my opinion.
Now, for Sterek, a 3-4 year age gap doesn't really bother me, but anything larger than that, when Stiles is still a teenager, is just too much. But, for Stiles and Derek specifically, I think 3-4 yrs is totally reasonable. See, the thing is, one of the main problems with most age gap relationships is that the older one has more life experience, but in this case, I'd argue Derek doesn't have much more life experience than Stiles does.
Derek's life was essentially put on hold for 5-6 years after the fire, he never had the chance to be a normal teenager, after that. Stiles might be younger, but both of them have lost people, they know what that kind of grief feels like, and I don't think there'd be any sort of power imbalance between them. They're equals. Stiles has no problem talking back to Derek and speaking his mind, and vice versa. I don't think Stiles was ever scared of Derek because he's a werewolf, I think he was scared because they thought Derek murdered someone. So, like, that's not a point of tension between them, it's not like Stiles (later on) is actually worried Derek will hurt him, and Derek (later on) trusts Stiles not to betray him or their pack.
It makes sense why they disliked each other at first, there were plenty of reasons to. Derek didn't trust Stiles because human=unsafe and Stiles was, admittedly, an asshole, and Stiles didn't like Derek because he was a weird stalker who he thought hurt his best friend.
But once they get past those things, I can totally see them being close friends, and more. Sure, there's plenty of things they disagree on, but they're also similar in a lot of ways, too. Like, this isn't an opposites attract situation (which I do love) because they're not opposites, even if it looks that way sometimes.
They both care way more than they let on, they both build walls and keep their friends out, they're both pretty jaded when it comes to trusting others (moreso later on, for Stiles) but help regardless. Oh, and they both blame themselves for shit that isn't their fault and take on huge amounts of responsibility even though they don't have to. I also think they're both nerds and probably have a good few shared interests. They'll argue on methodology but they value the same core things. Like, Loyalty? SO important, to both of them, same with trust. Plus, I think Derek's dry and blunt humor, and Stiles' sharpness and sarcasm, is a lovely combination.
So, in conclusion, I love them together and separately and think they're more compatible than they seem on the outside.
It's funny cuz I just made a post about how half the time I feel meh about Sterek, and then I get super carried away and rant about them for multiple paragraphs lmao. Basically I saw some character analysis posts (I adore everyone who makes those, btw, y'all are awesome!) and it revived my love for them.
(Again, if anyone disagrees with me or has anything to add, that's completely valid, this is just the way I interpret their characters, and I love hearing other ppl's thoughts!)