So, it's canon that Karkat is the youngest troll in his session, right? (Assuming that also makes Kankri youngest), is it ever stated who the oldest is, or would it be logical to assume their meteors arrived in order, making the Leijons oldest?

We have no idea if Karkat is the youngest. We can guess that based off it being his “birthday” when they played the game since that was the case with John. We don’t know the “birthdays” of any troll except Karkat.  Terezi and Aradia at least are older than him because they were also 6 while playing the game