Regarding Adult FS levels, I would assume most countries follow the Technical Requirements for International Adult Figure Skating competitions held under the authorization and support of the ISU, since these are also meant to be a recommendation for other Adult competitions held by ISU members. So it would make sense to base any test structure on them as well.
Fwiw though, we don't have test levels specifically for Adults in my home country, it's the same for everyone no matter your current age or the age you started at. Those tests are kind of meaningless for anyone who only competes at recreational and/or Adult competitions though, since you can just sign up for any competition level you want no matter your actual skill level, since you don't need to have passed a certain test to be able do so (this is also true for one of the biggest ISU sanctioned Adult competitions, the one in Oberstdorf).
And yes, single Axel has long been part of the Silver level. There's no 'official ISU level' below Bronze though, but most countries have included a version of their own called Pre-Bronze or sth. similar in their competition structure. This was necessary, since many skaters don't have sufficient skills to realistically compete in Bronze and the Bronze category is usually the one with the greatest number of participants and the biggest range of skill level anyway. So it made sense to split up the field and give skaters the opportunity to compete at a level that's more appropriate to their skills and have a more rewarding competition experience.
I don't like the idea of this, it feels patronising. Apparently the judging at some competitions can be at times less strict for Adults, but I'm not aware of any official rules for that, so it's probably just determined by the panel on a given day.
I remember one time, when we held our yearly club competition including Adult categories, a group of older Adults later on complained about their scores, because our panel didn't judge them by the, and I quote, "more lax Adult standard" that they expected based on how they were judged at other competitions. Things like single jumps not being completely around, sit spins not being low enough or the free leg in a camel spin not being held high enough.
I also remember one time watching the Bronze category at the Oberstdorf Adult competition a few years back, and I'm not exaggerating, 99 % of the skaters did a Toe-Waltz, but all of them were called as a successful single Toe Loop (I believe they're more strict now, though).
