IME that's not necessarily true. I have quite a few clients (in fact I'd say the majority, regardless of age) who cannot squat to parallel without their heels coming off the ground or their knees caving in or out due to poor ankle mobility when they first start training with me. Keep in mind that the term "parallel" is an objective one, comes from powerlifting, and means that the top of the knee and the crease in the hip are parallel with the floor. For most people there is actually not all that much difference in range of motion between a parallel squat and a "full/deep" squat.
Using plates or Oly shoes can be a nice quick fix and can allow for a more upright position while Oly squatting, but unless you are going to compete in Oly lifting, I'd work on your ankle mobility to the point where you are at least capable of squatting full ROM without the need to raise your heels. Whether you actually do a lot of squatting flat footed (I personally prefer either wrestling shoes or Vibrams 5 Fingers, barefoot is also awesome but most gyms won't allow it) is up to you, but the ability to do so should be a goal of yours if you want to keep your body functioning properly for the long haul.