In Dragon Ball, Mai is a member of Pilaf's Gang and was aftering the Dragon Balls ever since Goku was a boy (age 12).
In Dragon Ball Super, Mai appears to be young again and studying with Trunks. At the same time, Goku is already a grandpa.
What is Mai's real age?
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.
Toriyama hinted her age. In Dragon Ball Z: Battle of Gods, Mai tells her age as 41, and that would be the age she was in Dragon Ball, meaning she was over 80 years in Dragon Ball Super, and Future May is around 100 or more years old.
As to when they made that wish, the best placing would be the Android Saga before Cell appeared and Picollo and Kami fused. They turned themselves into toddlers or just very young children. In the alternative timeline, the dragon was only able to grant the wish for Mai as Picollo got killed by the Androids, both Shu and Emperor Pilaf died of old age, leaving Mai alone, sometime after the wish.
In Dragon Ball Super, she would be 10-11 years old due to her and the Pilaf Gang making that one wish to be young again. So she and Trunks would be about a year or a few months apart in age.
If you are asking about her real age, as in overall (since the Dragon Ball series), she would technically be older than Goku and even Bulma, since she was already a fully grown adult during the Dragon Ball series.
Although on a side note, if you look into Dragon Ball GT, she is/was an old woman while Trunks was a young man. So I don't know what happened there!
Let me speculate here.
Based on Wikia, Pilaf is 20 years older than Bulma, and it would convenient to assume that Mai is younger than Pilaf but older than Bulma. When Mai stated that she is 41 in Battle of Gods, I'd assume she refers her age before made young by Shenron. Thus 41 + 11 (age 767) = ~52 years which is 8 years older than Bulma, 12 years younger than Pilaf.
Make sense to me.
