Is it common to have eye floaters at the age of 13/14?

Uncommon floaters. No , this is not common at all. I suggest that you bring this child to an ophthalmologist for a complete dilated examination, to rule out any pathology in the retina.
Not really. But that does not mean that it is a problem to have floaters at a young age. Most people develop floaters in their 40s and beyond as the gel that fills the back of the eye starts liquifying. This causes small pieces of gel to float in the liquified portions. Since children have a tighter gel, it is less likely to occur. If there are suddenly a large amount of floaters, see an eye doctor.