I actually briefly considered this when I was writing The Light and The Blade, but if you check up on Xin's lore, the guy has to be old. He was rescued by Jarvan II, and currently serves under Jarvan III. While no numbers have been stated, we know that there is a Prince Jarvan IV, meaning Jarvan III has to be a decent age himself. What I'm trying to say is, there's been a lot of years between the start of Xin's story and the present day. There's no way he's under 40.
Meanwhile, looking at Lux I'd be surprised if she was even 20.
My mom is 50 now, yet people have mistaken her to be in her late thirties. She fools even those age guessers that you see sometimes around malls and such.