Enthusiasm the subject has much more to do with the learning than your age.

The only problem you will have is probably to be able to calculate simple things fast, like regular multiplication, addition and algebraic simplification. These are things that you can only learn with a lot of practice and time. (And this is basically all you learn before the age of 18)

New theory will probably not be a problem for you at all. In my experience, the theory is not an issue, but it is mostly the mathematical self esteem that you need. You NEED to believe that you can and will be able to understand the theory.