TL;DR: She would be born in between 1998 and 2000 making her 18-20 in Avengers: Infinity War but presumably closer to the older end of the scale to make her older in the previous films.
As far as I can tell there is no official information on Wanda's birth date but we can narrow down the range for why they could have been born.
She was ten when the bomb hit and blames Tony Stark
Pietro Maximoff: We were ten years old, having dinner, the four of us. When the first shell hits, two floors below, it makes a hole in the floor. It's big. Our parents go in, and the whole building starts coming apart. I grab her, roll under the bed and the second shell hits. But, it doesn't go off. It just...sits there in the rubble, three feet from our faces. And on the side of the shell is painted one word...
Wanda Maximoff: Stark.
Pietro Maximoff: We were trapped two days.
Wanda Maximoff: Every effort to save us, every shift in the bricks, I think, "This will set it off." We wait for two days for Tony Stark to kill us.
Avengers: Age of Ultron
Tony was CEO from 1992 to 2010
According to the SHIELD case files supplied with The Avengers DVD release Tony was born in 1970.
Click image to enlarge.
Tony is made CEO at 21 after his parents died and as they died in December 1991 this presumably happened in 1992 before his birthday.
Voice over: Then, the passing of a titan. Howard Stark's lifelong friend and ally, Obadiah Stane, steps in to help fill the gap left by the legendary founder, until, at age 21, the prodigal son returns and is anointed the new CEO of Stark Industries.
In 2010 Tony then hands over the CEO position to Pepper Potts.
Tony Stark: I hereby irrevocably appoint you chairman and CEO of Stark Industries effective immediately. Yeah, done deal. Okay?
Iron Man 2
All this together means that the twins would have been born from 1981 up to 2000 making them 15 to 34 in Avengers: Age of Ultron. Of course the twins could blame Stark still after he left the company but considering the move away from being a weapons manufacturer it seems unlikely this would have happened then.
Clearly they don't appear to be at either end of the scale in the films so we could narrow it down further assuming Clint is correct in saying that Wanda could go to High School.
Clint Barton: You gotta help me, Wanda. Look, you wanna mope, can go to high school.
Captain America: Civil War
High School age is from 13 to 18 and Civil War takes place in 2016 so from there we can calculate that should would have been born between 1998 to 2003. As we already have a range capping out in 2000 that means they would have been born from 1998 to 2000.
If she was born in 1998 that would make her 10 during the events of Iron Man which would tie up quite nicely with Stark's move away from being a weapon's manufacturer and keep it tied into the MCU so it seems possible this is when she was born.
Therefore, as of Avengers: Infinity War (just to make it easier due to the Blip), she would either be 18 or 20, presumably closer to the latter to make her age in the earlier films a bit higher.