I think my idea (and I drift back and forth nowadays a lot) is that these Ancient (all of the above) types and forms are accessible to all Digimon. Sometimes it's a requirement of age and experience, however, to evolve to a level where the data is that "pure". These Digimon forms are the most idealized and concentrated embodiment of specific Digimon species: Beasts, Dragons, Birds, Insects, etc. However, there are also The Ten. The Ancient-prefix Digimon. The Legendary ones. Those specific 10 can be just about anyone - they just stumble upon, or hunt down, or acquire in some way/shape/form the specific Ancient Elemental power - it's deity power, left over by beyond-Elder gods at the formation of the world.
Put it this way - Thor is a god, he's basically the current generation. Odin represents (along with his contemporaries) the older edge of the current generation. As great as Odin was, he's no Elder God; they were gods of his father's time: Bor. Bor, Izanagi, Cronos, Praamzius, Ra, Ahura Mazda...and the gods who seeded the 10 Ancient powers into the world were older even than that.
So the Ancient powers are uber-old.