Region 6 USAG
My kids are Region 6 and we are governed by USAG.
Mobility/Eligibility are usually done a couple of times per competitive season. A coach can also petition to the judges before the meet if they feel their gymnast is ready to mobilize. The judges will then judge the event on that merit and taking into consideration the coach's imput.
Meet fees vary by state and our Region does NOT charge a gate fee, artistic does. T/T is much more laid back and unstructured. If you are used to Artistic, it may seem like it's a 3-ringed circus.
There is no age max in our region, I think that goes all over as well. We have one judge who is in his 60's and still competes tramp.
All of our gymnasts qualified for Regionals and 3 for Nationals. My 2 did not attend Nationals, too far to travel.
One element of Tramp that isn't talked about very often is Synchro Tramp. Two gymnasts, same level, do a routine together. When it's done right, it's breath-taking to watch. Both of mine do, with different partners and when they do, the whole gym stops to watch, coaches included! Our teams have taken 1st place at each competition, including states and regionals and will receive banners at our gym for their accompliment.
I hope that helps answer some of your questions.