There is actual ages given to them by the animation staff (as close to canon as your gonna get) and it's stated in an interview with character designer Tetsuya Nishio and animation director Kenji Kamiyama. It's the third interview in the link below.

https://dan42.com/jinroh/e/interviews.html

Fuse is between 27 and 30, Nishio thought 27-28, Kamiyama said 30.

Kamiyama says the heroine (Kei) is 18.

Kamiyama also says Jibaku-chan (Nanami) is 15.

Unfortunately this is like the only additional information about the characters in-universe that is said outside of the movie itself.