Floaters can occur at any age, but they become more common as we get older due to imperfections that develop in the vitreous. Floaters occur within the eyeball, so when you try to move your eye to look at them they will move away because you are moving the eyeball, which contains the floaters. They can be detected during an eye examination by an optometrist or ophthalmologist looking inside the eyeball, or sometimes the patient will physically see on the floaters themselves.
Most floaters are normal and will rarely cause problems with vision or blindness. However, because floaters can rarely be caused by more serious problems, patients having floaters should see their eye health care professional to determine the cause. Particularly, if you notice a large change in your floaters, or if you get them with flashes of light, this could indicate more serious retinal problems and you should get this checked immediately.
When floaters are associated with a retinal problem an ophthalmologist will treat the retinal problem, and sometimes also remove the vitreous (and therefore the floaters) from the eye. However, if the floaters are not associated with another problem your optometrist or ophthalmologist will usually encourage you to try to put up with them as they will usually disappear with time. As the particles causing floaters are contained within the eye and are usually a normal part of the eye they cannot be prevented.