--- Begin quote from "Chaddy"
My question is why does the top string only go halfway up the neck?
Are all banjos tuned to Open G? And lastly, is there a set order to take in Ben's lessons? .
--- End quote
The 5th string needs to be shorter to produce the higher note. Just like fretting a string the note gets higher as the string gets shorter.
There are many different tunings for the banjo. Open G is the most popular for bluegrass/Scruggs style picking and the majority of tabs you run into will be in that tuning.
Welcome to the site!