From my own impression I'd have to go with time travel as our explanation of why Turlough was a serving officer when Malkon was an infant and their apparent ages later. I know that very idea has been proposed before, but I think this gives it additional weight. The war isn't over yet in KoD and Turlough was an adolescent 3 years ago. As for Lomand not being surprised at Turlough's age, I guess it's fill-in-the-blank on your explanation. I'm going to go with him knowing Turlough was traveling with a time traveler for the moment.
On a sidenote, it seems like there was a line about Turlough's father and brother and that he didn't know what had become of them (or indicated something other than the colony planet), but for the life of me, I can't recall the specifics. Can anyone else?
Now I can't promise I didn't miss a line or two and end up getting completely the wrong idea, but those are my thoughts.