Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.
jQuery 2.0 is out! The exclamation point was in order because it's a great release. It draws a line in the sand about browser support, no longer offering support for IE 8 and down. The library lost a little bit of weight doing that and likely makes it a bit more fun to work on for the devs =).
Because CodePen is often used for cross-browser testing, we're going to keep linking to jQuery 1.9.x.
The salient point being that there is nothing jQuery 2.0 can do that jQuery 1.9.x can't do, so we might as well give you the deepest support we can. The day that jQuery 2.x offers different functionality than 1.9.x, we'll start linking to that.
On production sites though, if you don't plan to support IE 8 and down, we wholeheartedly recommend you switch over if just for the size savings. We'll be switching CodePen soon.
If you really do want to pin a version of jQuery on a particular Pen, you can easily do that by linking the library as an external resource rather than picking it from the library dropdown. Here's the Google CDN list of library URLs for reference.