So at the beginning of the killing game they are all 16. However, as time progresses, the older ones pass their birthday. It's left up to interpretation *exactly* how long the killing game is, but each canon game (i.e. THH, GD, V3) lasts about 20 to 25 days. If that is applied to DH, assuming the killing game started in April as the Japanese school year does, then only Yosai would have turned 17, and that's *if* he even survived to see his 17th birthday. (However, you can headcanon the killing game to take place over whatever span of time you want, with the absolute minimum being two weeks given the amount of morning/night announcements there are).
Yosai: May 1
Mizuki: June 2
Haruko: July 20
Daichi: July 22
Emiko: August 7
Hayato: September 13
Aiko: September 24
Kyoji: September 26
Serena: 16 October 8
Arisa: November 12
Takayuki: December 17
Namika: December 22
Isoshi: February 2
Kat: February 24
Baiko: March 31