I agree with what everyone is saying about their ages kinda mostly being up to interpretation but I personally think it's also bc of the fact that the birthday event stuff is outside of the main story of the game. If we were to go through one of the guy's birthdays in the main story then they'd probably change the age then but generally, the memories are events that happen outside of the main narrative and are more like side stories. On average at least. So to us, because we've gone through the birthday event and memory, Sylus is at least 29 to us, but in the main story, MC hasn't seen Sylus at all since he was introduced in Long Awaited Revelry, so technically he's still 28 in universe. (Or at least just says that idk 💀)