Adult Gold leads to standard intermediate, or, in adult competition, Masters Intermediate, Masters Novice, or Championship Masters Intermediate/Novice (those levels get combined for the qualifying competitions, and all those levels have a minimum test level of standard intermediate). It's not an age thing, though. Any adult skater who has passed the intermediate test can compete in those levels. A 21-year-old who has passed the standard intermediate test goes straight into one of those Masters categories. Within the adult competitive categories we subdivide by age class if the number of entries warrants.
For Moves in the Field or pattern dance testing, on the other hand, Masters means you are age 50 or older. For example, you can take the intermediate (or higher) Moves test on the standard track, the adult track if you're at least age 25, or the masters track if you're at least age 50.
This is all spelled out in the Tests portion of the Rule Book, which is available for anyone to read on line at US Figure Skating.