Steph,
It's true that MDS is more common in the U.S. than aplastic anemia. You aren't in the age range where either disease is most commonly diagnosed, so that's not a clue. Only a bone marrow biopsy can determine if you have the dysplasia (bad structure or form) of the bone marrow that would distinguish MDS from AA. Doctors can't tell the difference from looking only at circulating blood.
How to know when blood counts indicate that treatment is needed is widely variable. Some people live just fine, symptom-free, while other people with the same counts suffer a reduced quality of life. Doctors' opinions also vary about when drugs are appropriate to treat low counts, or even the cutoff numbers under which you should get a transfusion.
Since you aren't having symptoms your watch-and-wait strategy makes sense. See the
Symptoms of MDS page symptoms that should be brought to the attention of your doctor. The symptoms of AA are basically the same.