Honestly, I'm thinking that majority of the characters are 15-18 years old since a lot of them are in high school (with the exceptions, of course, being Haiji and the college students). Since the game said Sena is a 2nd year and Tomose is a 1st year, that means they're around 15-16 years old as the average age for a 1st year in high school in Japan is 15. Japanese high schools only have 3 years, unlike the US where it's 4.
Honestly, I'm thinking that majority of the characters are 15-18 years old since a lot of them are in high school (with the exceptions, of course, being Haiji and the college students). Since the game said Sena is a 2nd year and Tomose is a 1st year, that means they're around 15-16 years old as the average age for a 1st year in high school in Japan is 15. Japanese high schools only have 3 years, unlike the US where it's 4.