Unlikely. Most mental functions diminish to some extent as we age. The first to go seems to be the remembrance of names and faces. Anyway, most slow down a little in mathematical ability, less so if math is an important part of their life. Exercise, even mental exercise, usually responds to frequent use. If you did not have a "learning disability" at a younger age, you probably dont have this now.
Getting older. As we age, our bodies do not respond as quickly as they used to in youth. Learning (or relearning) math or any other knowledge may be slower and less complete than before. If this inability is severe enough, consult a physician to see if a medical problem, neurologic or non-neurologic, may be responsible.