The astral princesses, as I like to call them, actually have a long lifespan (around 5000 years). They were created to end tensions between the three tribes by the queen Chrysanthema, who was the ruler before they arrives to equestria. They were "born" alicorns, not like the "ascended alicorns" that are Twilight and Cadance. They just received the elements that were missing to make them alicorns. For the CMC, I haven't really made solid headcannons on them.