In 2013, the Dragon Ball gave Trunks a new love interest in the form of Mai. However, in Dragon Ball Super, the two are shown having grown up together and developing a strong bond. That initially seems wholesome, but there's one thing fans can’t help but ask about: Mai's actual age.
In truth, Mai is an adult much older than Trunks; a wish on the Dragon Balls reverted Mai to a child around Trunks' age. But her actual age is a little more difficult to answer, especially considering how the timeline works in Dragon Ball Super. Determining Mai's real age will require an understanding of Dragon Ball's timeline, and where Mai falls in it.
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Mai's first given age comes from Dragon Ball Z: Battle of Gods. Before the events of the movie, the Pilaf Gang used the Dragon Balls to restore their youth, but they were made too young and turned into children. Despite that, when asked about her age, Mai thoughtlessly answers "41 in March." Because Battle of the Gods takes place in 778, roughly four years after the Buu Saga (Age 774), Mai's presumed year of birth is 737.
What's peculiar about Mai's given age is that it's around the same age as Goku. At this point in Dragon Ball continuity, Goku was presumed to have also been born in Age 737. Despite that, by the time they meet in Age 749, Goku looks no older than 12, which adds up, while Mai looks no younger than the mid-teen Bulma. It's possible Mai lied about her age, and she's actually a little older than she cares to admit.
Dragon Ball Super makes this question even harder to answer. Unlike the movie, this series begins mere months after the Buu Saga. While Mai's comment about being in her forties doesn't make it into the anime, if it holds true at this point, then her being older than Goku (who should still only be 37 in Age 774) is more believable. In any case, it's safe to assume that, chronologically, Mai is either in her late thirties or early forties by this point. As for Mai's age as a "child," she's probably close to Trunks', if not a little older.
When she's introduced at this age, she's shown to be a little taller than the boy. That changes by Future Trunks' time, but that's more likely due to the difference in how boys and girls develop in regard to height. If this is true, then Mai should be at least 12 by Age 778.
This is further backed by a bonus chapter of the DBS manga from 2016. In this chapter, the Pilaf Gang is shown wishing to be younger around the time Trunks was a baby. They themselves are reverted to babies by the wish. Assuming this part of the anime and manga continuity line up, it proves that Trunks and Mai are around the same age.
It's worth noting that, despite their physical ages, neither Trunks nor Mai look any older, despite entire years going by. Trunks has looked like he was 8 since Age 774. This makes sense in that year since he actually was 8 at the time. However, by the time of the Tournament of Power (Age 780), he should be at least 14. Even History of Trunks, which also took place in Age 780, knew to make Future Trunks look older based on his apparent age. Regardless, Trunks and Mai remain cute and marketable for DBS.
Whatever the cause of Trunks and Mai's lasting youth, they should be assumed to be around the same age following Mai's regression. She starts off in what should be her early forties but regresses to around 8-12 years old due to the wish on the Dragon Balls. This holds true whether Trunks and Mai met in Age 774 or Age 778. This doesn't explain why they still look 8 to 12 years old by Age 780, but that's how it is. At the very least, Dragon Ball Super: Super Hero seems like it will finally make these two look as old as they should be.