I had to find out so started a new save (it's still irritating that I have to wade through starting a new household each time I start a new game, so I can't just go into the game, choose a world, then choose a townie household if I just want to play townies) and at Long lifespan, Luna was a whole day younger than Hugo. At Normal, they both were set to age up in 13 days from teen, but she still appeared to be younger, because her green bar was smaller, so it looks like he's older by less than a full day of normal lifespan, but older nonetheless.
Well, mystery solved: Luna and Hugo may be "Irish twins" (meaning less than a full year apart in age) but they aren't literal twins.
But just re-read what @SenpaiCanberra wrote, and now I'm tempted to go back in and start a few new saves and see if they all come out the same (though it would take a lot, not just a few, to be reasonably certain of the result). How weird if that isn't the case! Age doesn't appear to change randomly for any other sim in the game. Does it?
