She's 10 years old.
Dragon Ball Wikia says
At some point before May 12th, Age 767, Mai and the other Pilaf Gang members managed to use the Dragon Balls to wish for youth.
In 767, Mai makes a wish, in the same year the androids appear, and Future Trunks follows them back. Present Trunks is one year old.
In 777, Trunks is 11 years old and future Trunks is 31 years old (he's from the year 797).
So if Mai's wish in 767 to be young turned her into a baby she's one year younger than Trunks, who was born in 766, so she would be ten years old in Dragon Ball Super in the year 777.
So in conclusion, Trunks was born in the year 766 year and Mai turned into a baby in the year 767. Current Trunks is 11 years old so Mai is 10 years old.