My approach at parsing this:

There's two point in time with two ages each:

M, M0, and A, A0

Then the sentence can be expressed as:

* M = M0 + X and A = A0 + X and (X is the time difference)

* M = 24 and ("Mary is 24")

* M = 2 * A0 and ("Mary is twice as old as Ann was")

* M0 = A ("Mary was as old as Ann is now")