I'd say 5-9 playing with rubber dinos is not too weird an age gap. But it's really unknown. Willi could be misleading Taylor. Think about Katie's circle including mostly people WIlli's age: Mon, Gina and Dana were all the same age as William and Leia right, being in the same year in school? At least that was the vibe I got - Mon and Dan graduated and got jobs, Dana in particular has been out long enough to make a quick fortune and buy rum, cigars and palatial apartmentsm aiming to get promoted to all time badass of the company. That doesn't happen on your first year out.
So I'd say th intended age gap is small, and Ocean just hasn't thought it through 100%.
Kata's seen as the young one.