I think an unreasonable amount about Xeno's extremely brief description of Senku to Stanley to identify him.
Mainly because he says Senku is 19.
But Senku isn't 19. He's 18.
Stan and Xeno have had their 4th birthdays after reviving already, but Senku's birthday is...earlier in the year, but because they revived in April, Senku's 4th birthday after revival hasn't quite happened yet; they're still about two months away.
So...Xeno doesn't know when Senku's birthday is, I guess.
Xeno is still kidnapped when Senku's 19th birthday actually happens, and you know that even their strike team isn't going to let Senku's birthday pass without acknowledgment whatever the situation. So kidnapped Xeno being forced to attend a Senku birthday party is actually hilarious. As a concept.
And that's gotta have been a deeply weird experience. Especially as Xeno did not know exact dates or years when he revived, so now it's time to realize the horrifying method via which the KoS has an accurate calendar, because in this one specific way, Xeno is less insane than Senku.
(I do think Xeno would have been able to cobble one together with knowledge of precession and constellation shifts, but it would have taken him a lot longer and I'm not sure he'd have ever quite gotten exact dates down)
But another detail from Xeno statement of Senkus age is:
Xeno knows how long Senku has been awake.
The series did that whole very cheesy, very emotional very Mirror Foils thing where they were petrified for the exact same amount of time and woke at the exact same date because the stone is worn away primarily by Brain Power and it indicates their intelligence/brain usage. So of course these two woke up first, and together, because they're Equals.
Like, Xeno and Senku don't know that (I think), but Xeno is not unaware of people waking up later. They put up a sign about it.
But Xeno just Knows Senku has been awake at least as long as he has, or really, exactly as long he has.
He really does believe in and respects the hell out of Senku. Maybe just as much as Byakuya does. Xeno was too busy to think about Senku, and he didn't immediately assume Japan was Senku, which I think he really could have. But barring that...yeah, I dunno, their relationship makes me emo, okay?