Damian is 14. Dick missed his 14th Birthday. Damian went from 10 to 13 during Rebirth and in the Robin series he turned 14. So it is explained Tim is the one with the sus age. He was 18 when Damian was 10 and now he's 16 while Damian is 14. Tim's aging backwards. Jon is 17 and has been 17 since Bendis aged him up 2 years ago
This still doesn't explain why in superman: son of kal Damian shows up and looks to be the exact same age as Jon (which like I'm presuming Jon is 18 now considering he's superman and attending college in his series but tbf I don't really know for sure he could still be 17) point is Damian ages at least 4 years between his own comic and Jon's comic which are set at the exact same time and in the same universe so all the bat's really have an issue with aging