Class A airspace is a fairly straight forward type of airspace to understand, it covers the entire globe, from 18,000′ msl or FL180 (flight level 180) up to and including FL600 (or 60,000′ msl).
Airspace above FL600 (60,000′ msl) reverts back to Class G (uncontrolled airspace). While this may change in the future, 40 years ago when the airspace limit was set at 60,000′ there were few airplanes that could fly that high, and no one believed that was going to change for a very long time (there is very little air up that high). Now in the age of commercial space travel, this is rapidly changing and we may see either a new class of airspace in the future or an extension to the vertical limits of the current Class A airspace.
Class A airspace exists everywhere over the continental United States, most of Alaska, and the airspace up to 12nm offshore of the United States. The is no Class A airspace over Hawaii.
Yes some rare aircraft can actually fly at 60,000′ and beyond. How high can they go? Well that’s classified, but here’s a fun story of a SR-71 Black Bird
Los Angeles Center reported receiving a request for clearance to FL 600 (60,000ft).
The incredulous controller, with some disdain in his voice, asked: “How do you plan to get up to 60,000 feet?”
The pilot (obviously a sled driver), responded: “We don’t plan to go up to it, we plan to come down to it…”