Wanda is probably a bit older than some people are thinking! I’ve seen the idea that she’s still a teenager pop up surprisingly often lately, but she’s definitely not a teenager, and she wasn’t one in Civil War.

Christopher Markus and Stephen McFeely, writers of Civil War and Infinity War, have said so in this podcast (timestamp: 7:53):

Q: Let’s keep talking about Wanda for a second. One of the things that’s going on with the film is that everyone’s kind of treating her like a kid. I mean, she’s literally sent to her room, for a big chunk of the movie. And it made me think, how old is she supposed to be? Because Elizabeth Olsen isn’t a teenager.

A: No.

Q: It makes me think it’s just a lack of experience. Because there’s a generational aspect to this story that I think is really interesting.

A: We’re not saying she’s 16. It is just a lack of experience, and a level of condescension frankly, on some of their parts. Like “let’s put her in her room”. But it’s also a certain level, I think, of fear, because they don’t understand her power. She doesn’t understand it either, but they can’t quantify it. So the more childlike you make her the less of a threat she is, in a way.

We haven’t ever gotten a specific age on her, but I’m guessing more like 25 around the time of Infinity War. I think the fact that the costumers have felt comfortable giving her such big leaps in the “age” from the very youthful AoU, to the more college aged CW, to mom jeans:

indicates to me that she was never supposed to be so young that these costume-based age jumps would seem odd. Notably, every other character in the MCU has not aged in terms of clothing styles except for Wanda.