matt age theory?
we know that pidge was just starting middle school in the flashback at the end of episode 2 of season 4 because she says that she has to go through middle school without Matt there (because he will be on the kerberos mission)
most middle schoolers start at age 11 (turning 12 during the year) meaning that in the flashback pidge would have been about 12 years old. in the pilot we learned it takes months to get all the way out to kerberos so that would have taken some time.
in the pilot we are shown pidge a year after the failed kerberos mission which would have been a year and some months after the flashback in season 4 allowing the time for the kerberos mission to launch, fail, and have a 1 year anniversary of that failure. therefor pidge would have been about 14 at the time of the pilot.
we also know that pidge is 15 now which means that in canon from the time of the flashback to season 4 pidge would have aged 3 years and matt would have aged 3 years as well.
with the assumptions that the garrison is a high school program, said high school program wouldn’t send a student who hasn’t completed the program on a mission, and that most high schoolers start at age 14 (turning 15 during the year) and graduate when they are 18, then when matt went on the kerberos mission he would have been at least 18 years old.
this is also supported by the fact that keith, hunk and lance are all 17/18 year olds and were still part of the program and keith was the top student in the class but still wasn’t chosen to go on a mission. none of them were. they were all still being trained in the pilot which leads to the idea that the garrison wouldn’t send people to space until they completed the program/graduated. so again matt would have been at LEAST 18.
if he was at least 18 when he went on the mission and 3 years have passed in canon since the flashback scene, that would mean that in season 4 matt would be at least 21 years old.