Midnight is a character from 'My Hero Academia', and while the series doesn't explicitly state his age, we can make some educated guesses. He's a Pro Hero and a teacher at U.A. High School, which means he's likely in his late 20s or early 30s. Most Pro Heroes in the series seem to start their careers in their early 20s, and Midnight has been around long enough to establish herself as a respected figure in the hero community. Her flamboyant personality and confidence also suggest she's not a fresh-faced rookie but someone with years of experience under her belt.
Looking at her interactions with other characters, she carries herself with the maturity of an adult who's seen a lot. She's also part of the same generation as other teachers like Aizawa and Present Mic, who are confirmed to be in their 30s. Given that, it's safe to assume Midnight is around the same age. Her design doesn't give away much either, as she maintains a youthful appearance, but her demeanor and role as a mentor to students like Class 1-A hint at her being older than she might look.
Midnight's age isn't a focal point in the series, but her character is more about her bold personality and her role as a hero. She's known for her provocative costume and her Quirk, 'Somnambulist', which allows her to put people to sleep with her aroma. Her age is just a small part of her larger-than-life presence in the 'My Hero Academia' universe. Whether she's in her late 20s or early 30s, she remains one of the most memorable and dynamic characters in the series.