Toboso Yana has explained that the characters are canonically aging in-game:
Twst follows a European/American-style system, so compulsory education is until the age of 16. NRC is considered a ‘high school/technical college,’ so everyone enters at 16.
In Japan high school enrollment is typically at age 15, but in this case, everyone is one year older. This means that if you celebrate your birthday during your second year of school, you become an adult (18 years old).
NRC follows the European/American-style system that begins in September. This is the reason why birthdays started in September.
It was briefly mentioned in the story, but Ace is what would be considered ‘late-born’ in Japan (born in September, the oldest of the first years), while Riddle is ‘early-born’ (born in August, the youngest in the second year).
In the game, the age/gender of the prefect are not set and there are events like birthdays…(laughs).
The characters don’t refer to each other’s ages, but on the official website they are listed as how old they were when the prefect was summoned.
NRC is essentially a technical school. Although there is what is uniformly called a middle school, the systems likely differ depending on a person’s country of origin, and there are also a fair number of students who are homeschooled.”
As a result Jamil is the oldest second-year student due to his September birthday, which has also been specifically confirmed by Yana.
Riddle being the youngest second-year and only one month older than Ace has been confirmed in the main story, as has Silver turning 18 years old in-game.
Twisted Wonderland Character Ages in the Main Story as of May:
Lilia: 700~ years old
Malleus: 178~ years old
Leona: 20 years old (July birthday)
Trey: 19 years old (October birthday)
Rook: 19 years old (December birthday)
Idia: 19 years old (December birthday)
Cater: 19 years old (February birthday)
Vil: 19 years old (April birthday)
Jamil: 18 years old (September birthday)
Jade: 18 years old (November birthday)
Floyd: 18 years old (November birthday)
Azul: 18 years old (February birthday)
Ruggie: 18 years old (April birthday)
Silver: 18 years old (May birthday) (not counting 400 years of sleep)
Kalim: 17 years old (June birthday)
Riddle: 17 years old (August birthday)
Ace: 17 years old (September birthday)
Jack: 17 years old (October birthday)
Sebek: 17 years old (March birthday)
Epel: 17 years old (May birthday)
Deuce: 16 years old (June birthday)
Exception: Ortho’s age is written as “N/A” in his official, in-game profile.
Twisted Wonderland Reference Note 