Well, they've been apart for several years now and Cass is an adult who can talk and read. Basically, she's a different person from the silent uneducated teenager that JP met back in the day. That's not saying that they'd jump into being a couple right away.
I guess it's like rediscovering old friends/acquaintances. After the catching up period ends, you start seeing them in a different light because well...they're different people now. People that you never would've thought to date back in high school (or middle school for some)...now years later, are very different individuals. So it's like meeting them for the first time and finding them more interesting.
I can see the age gap bothering JP more than it would Cass, actually. lol, eh, give'em time. Ten years from this, the age gap won't be such a big deal.