Okay, the premise is simple enough. Deelon, one of the two characters in this scene, is 21 by Earth standards. But, he is a Martian and his actual age as defined by him and Liv, the other character in the scene, is that he is about to hit 11. However, in the scene below, he is considering how Liv is making him feel like her son, who is a teenager by Earth standards... But from his point of view he doesn't see the age difference as being defined in the terms of a teenager. As a reader, bearing in mind that it was 7 scenes ago that I last had someone reference his age by Earthly standards, and that this scene is being told from his POV : Would you feel you prefer it as written above, or would you rather I tried to give another reference to the age difference between Humans and Martians? I ask because I really don't want to cross that line into patronizing the reader's intelligence.
I don't read very much sci-fi, I prefer fantasy, but I'd say to primarily focus on age in terms of Earth equivalent. If this is futuristic in setting then the Martians likely emigrated from Earth. So Earth customs and terminology would follow them. Also if communication is common between the two planets, everything would probably be dated by an Earth calendar and clock. And then translated to Martian time as needed. That's my take at least. It might be beneficial to listen to those who are bigger fans of sci-fi and will have experience with stories where this kind of dilemma was addressed.
Are your Martians a different species than Human or are they the children of Earth? If the former, then you might need more scenes illustrating the differences. The latter then would they use Earth date/time conventions, somewhat along the lines of Star Trek’s star date?
Depends on how long those 7 scenes are. This is one of those things that's probably best to deal with once the story is finished and you have people giving their opinion on the whole story. It's hard to say whether the reader will need a reminder without knowing what's between this and the first mention.
No, you pretty much called it the way it is in other sci-fi. Humans emigrate to Earth and take their measurements with them. No one is going to redefine the kilometer just because they're on another planet. Why do that for years or days (a small adjustment, but unnecessary), etc.?
Also consider that your readers are reading in (presumably) English, so using measurement standards that go along with that language is the most plausible. Unless it's quatloos that you're betting on a game of fizbin, then everything is different.