Older Dipper is not too different from 12 year old Dipper. He's become Fords apprentice after graduating high-school and getting his bachelors. He's also continued his vlog type videos called "Dippers guide to the unexplained," (these are Disney xd shorts!) which will occasionally feature ghost hunting. Although Dipper can also be laid back and silly, and is a generally happy guy, with Dib and Billy he tends to fall into the "straight man" role. He will usually react with frustration or exasperation to Dib and Billys foolish hijinks.

For Dippers voice, you can reference the voice he has in the show. Obviously, since he's not twelve, he would have a somewhat deeper voice, although not particularly deep. He should still have that awkward, slightly geeky edge. Here's a compilation of dipper being kind of sassy, a good look at his personality!

Note: I've provided a few lines, the third being from my second animation idea. I'm 100% ok with, and encourage improvisation on these lines. They don't need to be exact, be creative if you want! If you are feeling up for it or inspired, improvise your own lines too! Come up with whatever wacky thing you want - whatever you think Dipper might say. I'm super excited to hear your audition.