Well, for one, unlike Tom Holland and Letitia Wright, who were clearly chosen for youthful features that would continue to pass as very young, Elizabeth Olsen didn’t look like a teenager at the time of casting, even in the movie:
They already knew when Infinity War would be, and that the actress would be 27/28 at the time of filming it.
For two, even if she had meant to be 18/19 in Age of Ultron, I think the new creative team from Civil War on wanted her to be older than that.
And let me talk about the wardrobe they used to communicate that (bear in mind I’m talking Hollywood costuming technique–if I say “unkempt” I mean, Hollywood unkempt):
So, in Age of Ultron, her look is meant to communicate “young and rebellious”. This could have been expressed in a lot of different fashion styles, but some elements say those things in just about any of them: her hair is relatively unkempt, her make-up is smudged and dark, she wears lots of patterns and layers and her clothes are generally a little bit more worn and rough looking. This is less to do with her living in impoverished Sokovia than one might think–Pietro has a much more kempt style to him, and his clothing doesn’t suggest as much about his age as hers does.
This character could read as a teenager, this is true. But if the creative team of Civil War wanted her to continue to read so young, we would have continued to see the smudged make-up, etc. Instead:
The costumers are careful to give her something close to the same style she wore in AoU, but they went out of their way to age it up. Solid colors, her clothes are much nicer, her hair is highlighted and styled, her make-up is refined, the silhouette becomes more defined. The costuming is telling us “this is the same person, but more put together and defined”–matured.
In IW, we only know of two outfits that she wears (and likely the only two we’ll see her in), so these outfits have to say a lot. Her fashion sense is still a little grungy, but she appears to be wearing pants (!), less layers, less make-up. Her hair is not unkempt, but it not quite noticeably less styled then in CW.
This matches with what we see of her in the hotel room, and that .1 second shot of her in costume in Wakanda. The “less make-up” thing is really pointed especially.
The fact that she is wearing contemporary fashion at the time of IW’s shooting isn’t as meaningful. They styled her in a way that would read as older than her CW self and significantly older than AoU.
You might think "Well, sure, she did age after all”, but so has every other Avenger, and they haven’t been given costume-based age jumps. Even a character like Peter has stayed consistently dressed over the three movies we’ve seen him in–because it’s important that we continue to see him as a young teenager, even if he’s supposed to have matured between now and SMH.
And that’s what I’m getting at, if Wanda was meant to be very young and always read that way, her costuming would communicate this. In my opinion 22 is still too young for these jumps to have made sense, especially relative to the other Avengers who are no younger than 35 at this point.
Oh and quick note–someone mentioned that Wanda was wearing boyfriend jeans when I said “mom jeans” here–I think the line is a bit blurred between those two as distinct fashion styles, but IMO, boyfriend jeans are generally looser, lower cut, and straight-legged. The key is they look masculine in style. Wanda’s are tight, past her navel, and tapered. Definitively feminine. I don’t mean to say she’s meant to look dowdy (all the Avengers dress in something that can be seen as fashionable, even Steve) but she’s definitely meant to look older.
Finally–and this is 100% me being biased–if we want to see Billy and Tommy, our chances are better if she’s a bit older than 22.
Those Celestial Bodies 