For the most part, anyone can access basic levels (weak, not requiring large amounts of mana/ether) of magic. Magical ability tends to wane with age as well. A large population of sentient beings on the planet can access moderate, or medium levels of magic for daily use or other tasks. Few can tap into large amounts of mana/ether; those individuals are quite special.
Going into it a bit more, more recent generations of inhabitants of the world can more easily access magical reserves overall, regardless of the amount (small, medium, large, etc.). Earlier generations (and ancient beings) have greater difficulty accessing mana/ether and it is less of an innate skill as it is with current generations of inhabitants. So, to put it simply, the older you are, the harder it is to use magic. There are, of course, exceptions to this with those who can access large quantities of mana/ether from birth or training, and the odd creature here and there. The Emissaries of the World’s Will can wield large quantities of magic, but this is due to the nature of mana/ether in this world and where it comes from.