Predominantly the difference has to do with speed and power - adults, and older adults (masters are 50+) are judged to a slightly lower standard than a younger skater would be, because we generally just don't skate as fast, or on edges that are as deep. However, it's been reported fairly often that a lot of judges seem to have an idea of what a test should look like, and will judge the same regardless of age.
That said, it is your OPTION on the dance tests (and MITF above Intermediate) to choose to test as an adult or master, or to take the standard test - for some people it's a bit of a pride thing to pass a "standard" test, rather than opting for the "adult" option.
As an adult, if you choose to test standard track MITF and Freeskate, there is no difference in scoring through Juvenile - because we have our own track with a lower passing standard.